Warning "ISO C++ forbids variable-size array" with constant size array

Question

I have a c++ program in which constants are stored within a class. And somewhere else in the code I use one of these constants as an array size.

Here is a sample:

Constants.h

#ifndef CONSTANTS_H
#define CONSTANTS_H

class Constants
{
public:
    static const unsigned int C_BufferSize;
};

#endif

Constants.cpp

#include "Constants.h"

const unsigned int Constants::C_BufferSize(128);

main.cpp

#include "Constants.h"

int main()
{
    char buffer[Constants::C_BufferSize];
    return 0;
}

When I compile this code with -std=c++11 -pedantic I got the following warning:

main.cpp:5:37: warning: ISO C++ forbids variable length array ‘buffer’ [-Wvla]

I don't really understand the error since the size is a constant, my guess is that at compile time the size is unknown.

The error can be bypassed using a heap allocated table (allocated with new), but I'm forbidden to use dynamic memory allocation. Therefore I'm looking for another solution.

You could use an enum, or C++11 initializer (`{128}`), and I think you could even intialize in-class (`= 128`). — JorenHeit, Jan 15 '14 at 13:01
Unless you wanted to use said constant in an `#if` condition. — Simple, Jan 15 '14 at 13:11

Nawaz · Accepted Answer · 2014-01-15T12:34:49.690

2

The definition is required and searched for, at link-time. So yes, the size is unknown during the compilation phase.

You're write this:

class Constants
{
public:
    static const unsigned int C_BufferSize = 128; //initialize here
};

And then only provide the definition in .cpp file:

const unsigned int Constants::C_BufferSize; //no initialization here

But then it makes better sense to make Constants a namespace instead of a class:

namespace Constants  //NOTE : a namespace now
{
    static const unsigned int BufferSize = 128; 
};

Seems more natural to me.

edited Jan 15 '14 at 12:34

answered Jan 15 '14 at 12:29

Nawaz

353,942
115
666
851

After a few tests it seems that the definition in cpp file is not mandatory. – Mika Jan 15 '14 at 13:00
1

@Mika it is if you *odr-use* it by e.g. taking its address. If you take its address but don't define it in a .cpp file you should get a linker error (this is for a `static` data member, not a global). – Simple Jan 15 '14 at 13:07

score 0 · Answer 2 · answered Feb 28 '18 at 04:20

Warning reason - as mentioned by @nawaz already (variable size in array at compile time - might not be supported/allowed by all compilers).

can be attempted this way instead:

std::vector<char> buffer(Constants::C_BufferSize);

Typically the above issue would arise when doing some char* conversion to string (limited by actual data size in the char-buffer). Therefore in that case, this can be used something like;

std::string ConvertCharDataBySizeToString(const char* s, std::size_t size)
{
    std::vector<char> data(s, s+size);
    return ( std::string(data.begin(), data.end()) );
}

Reference/s here.

Warning "ISO C++ forbids variable-size array" with constant size array

2 Answers2

Linked