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

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; }

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