C++ Array call-by-reference with template

Question

I want to use call-by-reference with an array and found something on here which seemed to solve my problem. However I now changed it to this and I get the error message

"template argument deduction/substitution failed".

It works if I put

bool a[3];

but not with the variable m.

#include <assert.h>

template <typename T, int Size>
void dosth(T (&a)[Size])
{
    assert(Size > 2);

    a[2] = false;
}

int main()
{
    int m=3;
    bool a[m];
    dosth(a);
}

You should state your error message rather than "it doesn't work". — OMGtechy
– OMGtechy, Commented Nov 14, 2015 at 16:52
The bool a[m]; is known as a variable length array and not supported in C++. However, if you were to use const unsigned int for the capacity or #define for the capacity, it would be an array in local storage. — Thomas Matthews
– Thomas Matthews, Commented Nov 14, 2015 at 16:54

Thomas Matthews · Accepted Answer · 2015-11-14 16:55:06Z

6

The bool a[m]; is known as a variable length array and not supported in C++.

However, if you were to use const unsigned int for the capacity or #define for the capacity, it would be an array in local storage.

If you want an array whose length is determined at run-time, consider using std::vector.

answered Nov 14, 2015 at 16:55

Thomas Matthews

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2 Comments

Dúthomhas Over a year ago

This is a known issue with VLAs. Their current implementation (from what I have read elsewhere) is crufted-in such that they will break with things like this.

Lia Fiona Over a year ago

Thanks, I think this answers my question. This is just the first time I am using C++ and I'm used to working with array from Java. I'll consider using std::vector next time. Maybe I'll try it with const unsigned int.

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