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In the following program, I have added an explicit return statement in func(), but the compiler gives me the following error:

m.cpp: In function ‘constexpr int func(int)’: m.cpp:11:1: error: body of constexpr function ‘constexpr int func(int)’ not a return-statement }

This is the code:

#include <iostream> using namespace std; constexpr int func (int x); constexpr int func (int x) { if (x<0) x = -x; return x; // An explicit return statement } int main() { int ret = func(10); cout<<ret<<endl; return 0; }

I have compiled program in a g++ compiler using the following command.

g++ -std=c++11 m.cpp

I have added return statement in function, then Why I got above error?

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  • 1
    You also don't need that declaration. A definition is also a declaration, and in this case, it harms readability IMO. Commented Jul 14, 2017 at 7:37
  • 3
    @tadman What? Of course constexpr function return values can depend on their arguments. What would be the point otherwise? Commented Jul 14, 2017 at 7:37
  • 2
    And the more interesting use would be: const int a=func(10); so computing a const-expression based on constexpr. Commented Jul 14, 2017 at 7:38
  • @Angew How much code can you cram into a constexpr before the compiler refuses to compile-time evaluate it? Commented Jul 14, 2017 at 7:40
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    @tadman In C++11, a single return statement. Since C++14, the sky (and [dcl.constexpr] and Annex B :-) ) is the limit. Similar to how template metaprogramming works. Commented Jul 14, 2017 at 9:19

2 Answers 2

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Prior to C++14, the body of a constexpr function must consist solely of a return statement: it cannot have any other statements inside it. This works in C++11 :

constexpr int func (int x) { return x < 0 ? -x : x; }

In C++14 and up, what you wrote is legal, as are most other statements.

Source.

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21

C++11's constexpr functions are more restrictive than that.

From cppreference:

the function body must be either deleted or defaulted or contain only the following:

  • null statements (plain semicolons)
  • static_assert declarations
  • typedef declarations and alias declarations that do not define classes or enumerations
  • using declarations
  • using directives
  • exactly one return statement.

So you can say this instead:

constexpr int func (int x) { return x < 0 ? -x : x; } static_assert(func(42) == 42, ""); static_assert(func(-42) == 42, ""); int main() {}

Note that this restriction was lifted in C++14.

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