VS2010中的const问题

Question

I have the shortest question possible:

Why does this not work in VS2010?

string keyword("lookuptable");
const int kwSize = keyword.size();
char oldBuffer[kwSize+1];

It works perfectly in GCC. VS2010 tells me that

"expression must have constant value"

I am using Win32 console application / empty project.

I am using absolutely nothing special, just

#include <iostream> 
#include <fstream> 
#include <stdio.h> 
#include <string.h> 
using namespace std

and its a single main function in a cpp file.

Answer 1

The size of an array must be an integral constant expression, or ICE (which means that it must be known at compile-time). You can use a const int in an ICE, but only if its initializer is itself an ICE.

A function call, like keyword.size() is not usable in an ICE, so kwSize is not usable in an ICE.

If it "works perfectly" in gcc it is either due to a bug or a language extension of some sort.

In C++0x, some function calls can be used in integral constant expressions, but they must be constexpr functions and there are restrictions on their use. To the best of my knowledge, no compiler fully supports constexpr yet. In any case, std::string::size is not constexpr .

Answer 2

GCC has a language extension allowing variable-length arrays. Visual C++ does not. You must initialize stack-based arrays with a fixed, compile-time constant.

Answer 3

As others have mentioned, non-constant array bounds are a GCC extension (likely a side benefit of its C99 support - C99 does allow non-constant array bounds). If you want this in C++, you should use a vector :

std::vector oldBuffer(kwSize + 1);

To turn this into a char * , do:

&oldBuffer[0]

This, while not strictly speaking valid C++ prior to C++0x, works properly in every compiler I've come across so far. C++0x retroactively blesses this usage, and also provides a oldBuffer.data() equivalent.

Answer 4

keyword.size() is evaluated at runtime therefore it is not a compile time constant. Depending on the compiler it may not be allowed to use an array size that is depending on some runtime value.

Some further info about variable array length in gcc

Answer 5

the answer is there "expression must have constant value". The const has to be something resolved at the compile time.