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)])'

• skverma: I want to know the reason why this error is coming

Answer 1

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;
}

• skverma: Thanks my issue got fixed