# C++ Char pointer/array type ambiguity

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So I'm trying to design a function that acts differently, based on whether it is passed a const char/wchar_t array, or a const char/wchar_t*:

template<typename Char, size_t Length>
size_t stringLength(const Char(&pString)[Length])
{
return Length - 1;
}

template<typename Char>
size_t stringLength(const Char* pType)
{
return strlen(pType);
}

const char* pTest = "Test";
stringLength(pTest); // => should return 4
stringLength("Test"); // => should return 4 as well

The problem is that the last line doesn't compile, saying that the function-call is ambigous between both overloads, even though it correctly identifies the argument as "const char [8]", which works as intended if I remove the "const Char* pType" overload.

Now, why is this ambigous? As far as I understand it, the upper function should be a closer match to the argument list and thus be selected. Is there anything I have to/can do to make that work? (I'm on MSVC 2017)

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Correct me if I'm wrong, but it's ambiguous because char* and char[] are the same type.  Both of them are just pointers to the first character in an array of characters.

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5 minutes ago, trjh2k2 said:

Correct me if I'm wrong, but it's ambiguous because char* and char[] are the same type.  Both of them are just pointers to the first character in an array of characters.

Well, from what I understand, a size-qualified "char[X]"-array isn't exactly the same type as a char*.

For example, you can convert the char[X] to a char*, but not the other way around:

char array[4] = {};
char* pointer;

pointer = array; // works
array = pointer; // doesn't

Also the first function can't be called with char*, and will have the correct array-size if called with a char[X]. So all of this ad least made me belive that they are different types; though obviously the compiler assumes they are ambigous, maybe for the reason you wrote.

I might have another idea that I'm going to try out though, just remembered that there was a std-trait to find out if a type is an array & to get the arrays extent... though thats going to result in more messy template code, so if someone found an easier solution I'd still appreciate it

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The problem I think is not that you can't pass char[x] to a char*, but that you CAN pass "text" to both.

What are you trying to accomplish?  This doesn't look like a good way to check the length of a string.

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24 minutes ago, trjh2k2 said:

The problem I think is not that you can't pass char[x] to a char*, but that you CAN pass "text" to both.

Yes, this is true, yet from how I can see it this only happens via cast (char[x] => char*), so under normal overload resolution rules, I still don't see how it would be any different to:

void func(int x)
{
}

void func(float x)
{
}

func(0); // calls "func(int x)"

I mean you're obviously right about what happens, it just feels wrong to me :>

24 minutes ago, trjh2k2 said:

What are you trying to accomplish?  This doesn't look like a good way to check the length of a string.

Its actually being used as an optimization string-length generation as part of my custom StringView-class:

template<size_t Length>
constexpr BaseStringView(StaticString<Length> pString) : // const Type(&)[Length]
BaseStringView(pString, StringLength<Length>(pString))
{
};

template<size_t Length>
constexpr BaseStringView(DynamicString<Length> pString) : // Type(&)[Length] => prevents issues with user-handled char-buffers
BaseStringView(pString, StringLength(pString))
{
};

I know its technically not 100% safe, but I made sure that it doesn't break anything for me; and since I'm using a string-view I'm already in not-safe territory. As you can see I've got a second overload that gets called when I'm passing in an actual "char array[X];" that is filled from ie. an windows-API method. The actual reason why I'd need the "const char*" overload is that right now this would instead call the "const std::string&" overload, thus creating an unncessary copy & a dangling pointer (if the view is actually locally stored).
Not that it happens that often, most of my codebase has now been ported to use StringView & size-qualified strings, but there's always some places where this could still happen.

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As you said before, char[] and char* are just the same, but depending on what you want to do/need you have to cast. And yeah, you can cast from char[] to char* this way:

char array[4] = {'a','b','c','d'};
char* charPointer = nullptr;

charPointer = &array[0];

You have to point your pointer to the beginning of your char array, there is no direct assignment. You might overload operators if you really use that much char-pointer assignment.

Edited by NajeNDa

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Various fun hacks for this exist at StackOverflow.

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The reference trick in the StackOverflow site is the one I've seen several times over the years:

template<typename T> void f(T* const& c){ std::cout << "pointer\n"; }
template<typename T, size_t N> void f(T(&)[N]){ std::cout << "array\n"; }

Even though you as a programmer don't know, your compiler hitting the code can potentially know.  It works if the parameter being directly passed in is known to be a fixed-length array. If it goes through a single indirection to a pointer and the indirection isn't optimized a way, then the information is lost and the compiler will deduce it as a pointer. If it goes through an indirection and the indirection gets optimized away it can still deduce it correctly.

So even though that can work in some cases, it won't work in all cases after indirections.

The useful cases are almost non-existent.

As for the original problem here on the thread where you're trying to avoid taking the string length, that's not much of a benefit.  You're trying to simplify the interface, but instead you are adding complexity by having an additional entry.  Instead of having only one interface:  (buffer, size), you've now got two interfaces: (buffer, size) and (fixed-length-array).  If the writer knows they've got a fixed array they can use (buffer, sizeof(buffer)). If the writer knows they've got a more traditional buffer they can use (buffer, buflen). They know the single interface is there and they need to use it.

Imagine if the C language used that in their interfaces.  You'd have the current set of twenty-ish memory functions like memmov, memcpy, memcmp, and a duplicate version of all the functions for fixed-length arrays.

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I guess what I meant to say is that rolling your own ways to check string length like this reads to me like a code smell / design smell kind of scenario.  If you created the array, you already know the size, so you can pass it around if you need it.

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1 hour ago, Kylotan said:

Various fun hacks for this exist at StackOverflow.

41 minutes ago, frob said:

The reference trick in the StackOverflow site is the one I've seen several times over the years:


template<typename T> void f(T* const& c){ std::cout << "pointer\n"; }
template<typename T, size_t N> void f(T(&)[N]){ std::cout << "array\n"; }

Ah, yeah, thats what I've been looking for!

41 minutes ago, frob said:

The useful cases are almost non-existent.

24 minutes ago, trjh2k2 said:

I guess what I meant to say is that rolling your own ways to check string length like this reads to me like a code smell / design smell kind of scenario.  If you created the array, you already know the size, so you can pass it around if you need it.

Well, I should have been a bit more specific about my use-case: As I've mentioned I'm using my own StringView class, akin to std::experimental::basic_string_view.

Now that means that functions may have a signature as such:

bool Node::HasNode(sys::StringView strName) const
{
return m_mNodes.count(strName) != 0;
}

where it would have been eigther "const std::string&" (for me), or possible "const char*" / "const char*, size_t" before. This has many benefits, as such std::string_view has been proposed, but thats not the point of this post. Now in my code, I might use those functions as such:

const auto strName = node.Attribute("name")->GetValue();
widget.SetName(strName.ToString());

const auto isVariable = node.HasNode("IsVariable");
widget.SetIsVariable(isVariable);

const auto visibilty = core::VariableLoader::FromAttribute<Visibility>(node, "Visibility");
widget.SetVisibility(visibilty);

const auto isEnabled = !node.HasNode("Disabled");
widget.SetEnabled(isEnabled);

Not the every function above takes a sys::StringView. And thats pretty much where I applied my optimization. std::string_view would take a const char*, and call strlen. My StringView-constructor can take a static char-array, and directly deduce the size from this - thats the reason why I don't wanna do it by hand even though I technically "know" the strings size, its simple convenience so that I can call all those functions with string literals, but without having to take a copy or determine the size.

41 minutes ago, frob said:

As for the original problem here on the thread where you're trying to avoid taking the string length, that's not much of a benefit.  You're trying to simplify the interface, but instead you are adding complexity by having an additional entry.  Instead of having only one interface:  (buffer, size), you've now got two interfaces: (buffer, size) and (fixed-length-array).  If the writer knows they've got a fixed array they can use (buffer, sizeof(buffer)). If the writer knows they've got a more traditional buffer they can use (buffer, buflen). They know the single interface is there and they need to use it.

As you should see in my explanation, the function I proposed isn't really going to be part of an interface, its just an additional constructor for my StringView-class that internally calls it. I don't know if that makes it any better in your book, but I do see a compelling case for handling string-literals the way I do. Also the purpose of StringView is to offer a unified interface from many types (std::string, const char*, const char*+size) to a single const char*, size_t-pair. So I'd say my general notion is not totally wrong - the only difference I make is instead of treating every "const char*" as a nul-terminated string, I'm making a differentiation between static string-literals as part of a small optimization.

41 minutes ago, frob said:

Imagine if the C language used that in their interfaces.  You'd have the current set of twenty-ish memory functions like memmov, memcpy, memcmp, and a duplicate version of all the functions for fixed-length arrays.

Sure, adding 3-4 overloads for the same functions is surely overkill, I agree on that (in my case I should have mentioned how its intented to being used), but since we are talking about C-API functions - as you can read in my other thread:

there's actually a lot of issues going forward with modern C++ now that most C-style API functions only take nul-terminated C-strings; which wasn't a problem before but now with string_view this is actually limiting its usefulness. So I'd personally rather have atoi(const char*) and atoi(const char*, size_t) than being forced to make sure my strings are nul-terminated... but I thankfully don't have to support a large userbase with my API, so my expertise in that regard is rather limited.

EDIT: Anyways, the suggested "tricks" seem to work, even though for some reason I have to add a template type to my template-class ctor for it to work:

template<typename Type>
class StringView
{
template<typename Char, CheckIsCharPointer<Char> = 0>
BaseStringView(const Char* const& pString) : // still ambigous if I just use "Type" directly
BaseStringView(pString, StringLength(pString))
{
};

template<size_t Length>
constexpr BaseStringView(DynamicString<Length> pString) :
BaseStringView(pString, StringLength(pString))
{
};
}

But the problem seems solved, so thanks to all for helping me solve the problem I'm still rather happy to discuss the issues revolving around this; I just recently started to work with string_view so its certainly good to get more input on it.

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To meet those three requirements, I've come up with the following solutions, all of which have significant drawbacks.
Static Policy-Based Allocators
I originally built this off of this talk.
Examples;
struct AllocBlock { std::byte* ptr; size_t size; }; class Mallocator { size_t allocatedMemory; public: Mallocator(); AllocBlock Allocate(size_t size); void Deallocate(AllocBlock blk); }; template <typename BackingAllocator, size_t allocSize> class LinearAllocator : BackingAllocator { AllocBlock baseMemory; char* ptr; char* end; public: LinearAllocator() : baseMemory(BackingAllocator::Allocate(allocSize)) { /* stuff */ } AllocBlock Allocate(size_t size); }; template <typename BackingAllocator, size_t allocSize> class PoolAllocator : BackingAllocator { AllocBlock baseMemory; char* currentHead; public: PoolAllocator() : baseMemory(BackingAllocator::Allocate(allocSize)) { /* stuff */ } void* Allocate(); // note the different signature. void Deallocate(void*); }; // ex: auto allocator = PoolAllocator<Mallocator, size>; Advantages:
SFINAE gives me a pseudo-duck-typing thing.  I don't need any kind of common interfaces, and I'll get a compile-time error if I try to do something like create a LinearAllocator backed by a PoolAllocator. It's composable. Disadvantages:
Composability is type composability, meaning every allocator I create has an independent chain of compositions.  This makes tracking memory usage pretty hard, and presumably can cause me external fragmentation issues.  I might able to get around this with some kind of singleton kung-fu, but I'm unsure as I don't really have any experience with them. Owing to the above, all of my customization points have to be template parameters because the concept relies on empty constructors.  This isn't a huge issue, but it makes defining allocators cumbersome. Dynamic Allocator Dependency
This is probably just the strategy pattern, but then again everything involving polymorphic type composition looks like the strategy pattern to me. 😃
Examples:
struct AllocBlock { std::byte* ptr; size_t size; }; class Allocator { virtual AllocBlock Allocate(size_t) = 0; virtual void Deallocate(AllocBlock) = 0; }; class Mallocator : Allocator { size_t allocatedMemory; public: Mallocator(); AllocBlock Allocate(size_t size); void Deallocate(AllocBlock blk); }; class LinearAllocator { Allocator* backingAllocator; AllocBlock baseMemory; char* ptr; char* end; public: LinearAllocator(Allocator* backingAllocator, size_t allocSize) : backingAllocator(backingAllocator) { baseMemory = backingAllocator->Allocate(allocSize); /* stuff */ } AllocBlock Allocate(size_t size); }; class PoolAllocator { Allocator* backingAllocator; AllocBlock baseMemory; char* currentHead; public: PoolAllocator(Allocator* backingAllocator, size_t allocSize) : backingAllocator(backingAllocator) { baseMemory = backingAllocator->Allocate(allocSize); /* stuff */ } void* Allocate(); // note the different signature. void Deallocate(void*); }; // ex: auto allocator = PoolAllocator(someGlobalMallocator, size); There's an obvious problem with the above:  Namely that PoolAllocator and LinearAllocator don't inherit from the generic Allocator interface.  They can't, because their interfaces provide different semantics.  There's to ways I can solve this:
Inherit from Allocator anyway and assert on unsupported operations (delegates composition failure to runtime errors, which I'd rather avoid).   As above:  Don't inherit and just deal with the fact that some composability is lost (not ideal, because it means you can't do things like back a pool allocator with a linear allocator) As for the overall structure, I think it looks something like this:
Memory usage tracking is easy, since I can use the top-level mallocator(s) to keep track of total memory allocated, and all of the leaf allocators to track of used memory.  How to do that in particular is outside the scope of what I'm asking about, but I've got some ideas. I still have composability Disadvantages:
The interface issues above.  There's no duck-typing-like mechanism to help here, and I'm strongly of the opinion that programmer errors in construction like that should fail at compile-time, not runtime. Composition on Allocated Memory instead of Allocators
This is probably going to be somewhat buggy and poorly thought, since it's just an idea rather than something I've actually tried.
Examples:
struct AllocBlock { void* ptr; size_t size; std::function<void()> dealloc; } class Mallocator { size_t allocatedMemory; public: Mallocator(); AllocBlock Allocate(size_t size) { void* ptr = malloc(size); return {ptr, size, [ptr](){ free(ptr); }}; } }; class LinearAllocator { AllocBlock baseMemory; char* ptr; char* end; public: LinearAllocator(AllocBlock baseMemory) : baseMemory(baseMemory) {end = ptr = baseMemory.ptr;} AllocBlock Allocate(size_t); }; class PoolAllocator { AllocBlock baseMemory; char* head; public: PoolAllocator(AllocBlock baseMemory) : baseMemory(baseMemory) { /* stuff */ } void* Allocate(); }; // ex: auto allocator = PoolAllocator(someGlobalMallocator.Allocate(size)); I don't really like this design at first blush, but I haven't really tried it.

"Composable", since we've delegated most of what composition entails into the memory block rather than the allocator. Tracking memory is a bit more complex, but I *think* it's still doable. Disadvantages:
Makes the interface more complex, since we have to allocate first and then pass that block into our "child" allocator. Can't do specialized deallocation (i.e. stack deallocation) since the memory blocks don't know anything about their parent allocation pool.  I might be able to get around this though.
I've done a lot of research against all of the source-available engines I can find, and it seems like most of them either have very small allocator systems or simply don't try to make them composable at all (CryEngine does this, for example).  That said, it seems like something that should have a lot of good examples, but I can't find a whole lot.  Does anyone have any good feedback/suggestions on this, or is composability in general just a pipe dream?

• Hi
I’ve been working on a game engine for years and I’ve recently come back to it after a couple of years break.  Because my engine uses DirectX9.0c I thought maybe it would be a good idea to upgrade it to DX11. I then installed Windows 10 and starting tinkering around with the engine trying to refamiliarise myself with all the code.
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There’s some relatively complex processing going on each time the mouse moves - the road either side of the control point(s) being moved, is reconstructed in real time so you can position and bend the road precisely. On my previous OS, which was Win2k Pro, this worked really smoothly and in release mode there was barely any slow down in frame rate, but now it’s unusable. As part of the road reconstruction, I lock the vertex and index buffers and refill them with the new values so my question is, on windows 10 using DX9, is anyone aware of any locking issues? I’m aware that there can be contention when locking buffers dynamically but I’m locking with LOCK_DISCARD and this has never been an issue before.
Any help would be greatly appreciated.

• I'm writing a small 3D Vulkan game engine using C++. I'm working in a team, and the other members really don't know almost anything about C++. About three years ago i found this new programming language called D wich seems very interesting, as it's very similar to C++. My idea was to implement core systems like rendering, math, serialization and so on using C++ and then wrapping all with a D framework, easier to use and less complicated. Is it worth it or I should stick only to C++ ? Does it have less performance compared to a pure c++ application ?

• Hi guys, I'm trying to learn this stuff but running into some problems 😕
I've compiled my .hlsl into a header file which contains the global variable with the precompiled shader data:
//... // Approximately 83 instruction slots used #endif const BYTE g_vs[] = { 68, 88, 66, 67, 143, 82, 13, 236, 152, 133, 219, 113, 173, 135, 18, 87, 122, 208, 124, 76, 1, 0, 0, 0, 16, 76, 0, 0, 6, 0, //.... And now following the "Compiling at build time to header files" example at this msdn link , I've included the header files in my main.cpp and I'm trying to create the vertex shader like this:
hr = g_d3dDevice->CreateVertexShader(g_vs, sizeof(g_vs), nullptr, &g_d3dVertexShader); if (FAILED(hr)) { return -1; } and this is failing, entering the if and returing -1.
Can someone point out what I'm doing wrong? 😕

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