cpp basic but easy confused question
Author
What's the difference between the following two functions:
returnType functionName const(arguments); //(1)
and
returnType functionName (arguments) const; //(2)
I know the (1) will allow the const instances to invoke the function. What does the (2) mean? Are these two functions have to be defined (or decleared) inside of a class? Thanks in advance.
http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.10
In short, you promise that calling the function keeps the state of the class instance unchanged. For instance, you'll often see this in simple property get-functions:
Edit:
My bad. I didn't spot the const in (1) :/
(2) still holds ;)
In short, you promise that calling the function keeps the state of the class instance unchanged. For instance, you'll often see this in simple property get-functions:
class Enemy{private: int health;public: // correct int getHealth() const { return health; } // wrong void kill() const { health = 0; }};Edit:
My bad. I didn't spot the const in (1) :/
(2) still holds ;)
Quote:Original post by daviddiligent
What's the difference between the following two functions:
returnType functionName const(arguments); //(1)
and
returnType functionName (arguments) const; //(2)
I know the (1) will allow the const instances to invoke the function. What does the (2) mean? Are these two functions have to be defined (or decleared) inside of a class? Thanks in advance.
(2) will allow the function to be invoked on a const instance.
(1) is not legal C++
As above, (1) is illegal. (2) means that the 'this' pointer is of type 'const T* const' rather than just 'T* const' (e.g., you can invoke the method on instances of 'const T').
Author
Thanks for WanMaster's reply. I remember the usage of (2).
But, I am sure that Both "jpetrie" and "Nitage" are wrong, because I just saw an example explaining that the (1) is for the use of const instances. And it is also legal. The usage of (2) is like what WanMaster's explanation
But, I am sure that Both "jpetrie" and "Nitage" are wrong, because I just saw an example explaining that the (1) is for the use of const instances. And it is also legal. The usage of (2) is like what WanMaster's explanation
Author
Quote:Original post by rip-off
Have you tried it in a compiler?
I saw it in a book. So I believe it's right. I can't try it now. Can you try it?
Quote:Original post by daviddiligentQuote:Original post by rip-off
Have you tried it in a compiler?
I saw it in a book. So I believe it's right. I can't try it now. Can you try it?
We could, but we already know what's going to happen. I think you need a new book.
Edit: O, what the heck...
MSVC:Compiling...1>Main.cpp1>e:\dev\temp\main.cpp(4) : error C2143: syntax error : missing ';' before 'const'1>e:\dev\temp\main.cpp(4) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int1>e:\dev\temp\main.cpp(4) : error C2062: type 'int' unexpected1>e:\dev\temp\main.cpp(5) : error C2334: unexpected token(s) preceding '{'; skipping apparent function body1>Temp - 4 error(s), 0 warning(s)
Are you sure you didn't misread?
These are both legal ways of declaring const instances. Furthermore
is a way of declaring a method that accepts a const instance of something. However, your syntax
isn't legal standard C++. See section 8.4.2 of the standard, where it states that the post-return-type portion of the function declaration must look like, basically, "name ( parameters ) optional-cv-qualifier optional-exception-specifier." It's not legal to have the cv-qualifier (const, in this case) between the name and the parameters.
const int x = ...;int const x = ...;These are both legal ways of declaring const instances. Furthermore
int method(const T ... );is a way of declaring a method that accepts a const instance of something. However, your syntax
int method const ( ... );isn't legal standard C++. See section 8.4.2 of the standard, where it states that the post-return-type portion of the function declaration must look like, basically, "name ( parameters ) optional-cv-qualifier optional-exception-specifier." It's not legal to have the cv-qualifier (const, in this case) between the name and the parameters.
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