The body of constexpr function not a return-statem

问题:

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?

回答1:

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.

回答2:

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.