CODEWITHSUNDEEP
c++c++11constexpr
jww
jww

Reputation: 102296

Min/Max/Step function and "body of constexpr function ... not a return-statement"

I'm catching an error when trying to decorate with consexpr:

$ g++ -std=c++11 test.cxx -o test.exe
test.cxx: In instantiation of ‘static constexpr unsigned int MinMaxStep<min, max
, step>::ValidValue(unsigned int) [with unsigned int min = 10u; unsigned int max
 = 100u; unsigned int step = 10u]’:
test.cxx:22:40:   required from here
test.cxx:16:5: error: body of constexpr function ‘static constexpr unsigned int 
MinMaxStep<min, max, step>::ValidValue(unsigned int) [with unsigned int min = 10
u; unsigned int max = 100u; unsigned int step = 10u]’ not a return-statement
     }
     ^

All of the values used in the problem function are template parameters. The values don't change after the file is saved.

Is it not possible to express this as a constexpr function?

If I am doing something wrong, then what is it? How do I modify ValidVaue into a constexpr function?

$ cat -n test.cxx
 1  #include <string>
 2  #include <iostream>
 3  
 4  template <unsigned int min, unsigned int max, unsigned int step>
 5  class MinMaxStep
 6  {
 7  public:
 8      static constexpr unsigned int Min() { return min; }
 9      static constexpr unsigned int Max() { return max; }
10      static constexpr unsigned int Step() { return step; }
11      static constexpr unsigned int ValidValue(unsigned int v)
12      {
13          if (v <= min) { return min; }
14          else if (v >= max) { return max; }
15          return (v+step-1) - ((v+step-1)%step);
16      }
17  };
18  
19  int main (int argc, char* argv[])
20  {
21      MinMaxStep<10, 100, 10> mms;
22      unsigned int x = mms.ValidValue (18);
23      std::cout << "value " << x << std::endl;
24  
25      return 0;
26  }

Upvotes: 2

Views: 1408

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409196

The rules for constexpr functions were very strict in C++11. For example there could be only a return statement, nothing else. The rules were relaxed considerably in C++14.

See e.g. this constexpr reference for more information.

There are two ways for you to solve your problem: The easiest is to use C++14 instead (change compiler flag to use -std=c++14). The other solution is to refactor your ValidValue function to only have one single statement, a return statement, using the ternary operator.

Upvotes: 5

Related Questions