compilation error while using constexpr

Question

#include <stdio.h>
#include <iostream>

using namespace std;

template <typename T, int N>
int ReturnArraySize(T (&arg1)[N]) {
    return N;
}

constexpr int ReturnTheSum(int arg1, int arg2) {
    return arg1 + arg2;
}

int main(int argc, char **argv)
{
    int arr1[20];
    int arr2[ReturnArraySize(arr1)];
    int arr3[ReturnTheSum(ReturnArraySize(arr1), ReturnArraySize(arr2))];

    return 0;
}

When I compile the code, I get the following error:

/root/Documents/C++11_Fundamentals/ConstExprRelatedFunc/main.cpp:19:67: error: no matching function for call to 'ReturnArraySize(int [(<anonymous> + 1)])'

I want to know the reason why this error is coming

skverma
– skverma

2016-03-31 15:35:50 +00:00
Commented Mar 31, 2016 at 15:35 — skverma
– skverma, Commented Mar 31, 2016 at 15:35

Piotr Skotnicki · Accepted Answer · 2016-03-31 15:50:49Z

2

Because ReturnArraySize is not marked as a constexpr function, arr2 becomes a VLA (variable-length array, a GCC extension, not part of the C++ standard), which cannot be queried for its size at compile time (i.e., deduced by a function template).

You can fix this by making ReturnArraySize a constexpr:

template <typename T, int N>
constexpr int ReturnArraySize(T (&arg1)[N]) {
//~~~~~~^
    return N;
}

answered Mar 31, 2016 at 15:50

Piotr Skotnicki

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1 Comment

skverma Over a year ago

Thanks my issue got fixed

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