What are your views on naming identical functions that do the same thing, with the only difference being whether they operate on the variable passed in (by reference), or operate on a copy of the variable (passed in by const reference) and returning it?

In my current code, I want to have two functions with the same purpose, but different usages. (both functions I have in a "String::" namespace)
I have:
std::string ReplaceAll(const std::string &str, ....);

I also have:
void ReplaceAll(std::string &str, ...);

They do the exact same thing (takes a string to operate on, a string to find, and a string to replace it with), but I use them in different ways.
I have more than just 'ReplaceAll()', I want multiple versions of several different functions. (Since I can always implement one function by calling the other, it doesn't introduce code bloat, and bug-fixes or optimizations to one automatically effect the other).

The first is useful for passing as parameters, such as this type of situation: (Supposing ToLower() is the function we are talking about)
map[ String::ToLower(key) ] = value;

It works well. If it didn't return a value, I'd have to do this:
std::string lowercaseKey = key;
String::ToLower(lowercaseKey);
map[ lowercaseKey ] = value;

That makes code less clear when doing a series of operations, because you have boilerplate lines for every operation of the function, instead of just having each operation be self-explanatory.

However, when I'm operating on a single value, or a series of values, I don't want to go like this:
str = Operation(str);
str = SecondOperation(str);
str = OtherOperation(str);
str = NextOperation(str);

I would much rather do the following, for code readability:

Operation(str);
SecondOperation(str);
OtherOperation(str);
NextOperation(str);


But I can't have two different versions of the same function named the same thing, for two reasons:
1) It'd be confusing to read. Which version am I calling here, version A, or version B?
2) C++ wouldn't allow it, even if I wanted it - the function parameters are too identical so the function names would collide.

So what would you call the two functions to distinguish them?
I was thinking something like,
ReplaceAllOn(str, ...) //Operates 'on' str, and returns void.
ReplaceAllFrom(str, ...) //Operates on a copy of str, and returns the copy.

Another possibility is using pointers for the 'operate on' version of functions, letting the & symbol indicate what's going on.
ReplaceAll(&str, ...)
str = ReplaceAll(str, ...)


However, I prefer using references when I know the variable address won't change during the function, and there's still the possibility of accidentally going:
ReplaceAll(str, ...) //Returning a copy, and having no effect, because you forgot the '&' so it called the wrong function. Hard to spot mistake when scanning code visually.

What I'd really love is a compiler error if I'm using the wrong function, but I can't think of a way to make that happen.
Blah = OperateOn(blah); //Compiler error: Blah = void is invalid. (This error already works)
OperateOnCopyOf(blah); //Compiler error: You're not assigning the return value to anything, you moron! (Sadly, this error my compiler doesn't inform me of)

Thoughts?

+1 for the _copy suffix. I would argue that it is the most idiomatic "c++" way of doing things, it's the approach taken by both the standard library and boost. As an example, take a look at the std::string member functions, the name of the function is the operation, and they are all mutating:

http://www.cplusplus.com/reference/string/string/replace/

So I would expect the default behaviour of a function to be mutating, unless the name of the function specifies otherwise.

I agree that the convenience of the copy version and the fact that it takes next to no work to implement make it worth the while.

For some inspiration you might want to take a look at the boost string algorithms:

http://www.boost.org/doc/libs/1_48_0/doc/html/string_algo.html

The thing std::string's functions having '_copy()' functions is because it's part of a class. My functions are free-floating - there's a difference in expected usage.

Mutating behavior should be the less preferred version

Why should it be less-preferred? I get the benefits of compiler-errors if I mistake it for the copy version ("Warning: Can't assign 'void' to 'std::string'"), but not vice-versa. Plus, it doesn't bother creating unneeded copies that it just discards. It's safer and faster, so why not prefer it?

What are your thoughts about using namespaces for this purpose?

string = String::Copy::ReplaceAll(string);
String::Mutate::ReplaceAll(string);

Or maybe having the mutate version just be String::ReplaceAll() and the copy versions of all the string functions, being the specialized versions, exist in String::Copy::ReplaceAll().