在C ++宏“ int val = rand（）”中创建新值

Question

I'm trying to create C++ macro which can do something while condition under "if" is checking.

My macro works in most cases, but I have problem when under if is created temporary value. Of course I can move creating of new value before "IF", but it is bothersome is I have to do in many places in code.

It is possible to get work "IF(int val = rand())"?

This code have no sense, but shows my problem.

#include <iostream>
#include <cstdlib>
#include <ctime>

bool printIf(int line, bool val)
{
  std::cout << "Line: " << line << std::endl;
  return val;
}

#define IF(x) if( (x) ? printIf(__LINE__, true) : printIf(__LINE__, false) )

int main()
{
  srand(time(NULL));
  //int val;
  //IF (val = rand())
  IF(int val = rand())
  {
    std::cout << "val = " << val << std::endl;
  }
}

Errors while compilation:

$ g++ main.cpp && ./a.out
main.cpp: In function ‘int main()’:
main.cpp:18:6: error: expected primary-expression before ‘int’
   IF(int val = rand())
      ^
main.cpp:11:20: note: in definition of macro ‘IF’
 #define IF(x) if( (x) ? printIf(__LINE__, true) : printIf(__LINE__, false) )
                    ^
main.cpp:18:6: error: expected ‘)’ before ‘int’
   IF(int val = rand())
      ^
main.cpp:11:20: note: in definition of macro ‘IF’
 #define IF(x) if( (x) ? printIf(__LINE__, true) : printIf(__LINE__, false) )
                    ^
main.cpp:22:1: error: expected ‘)’ before ‘}’ token
 }
 ^
main.cpp:22:1: error: expected primary-expression before ‘}’ token

Answer 1

I belive the problem is that you are wrapping the variable decleration in () . If I remove the parentheses then the code compiles:

#include <iostream>
#include <cstdlib>
#include <ctime>

bool printIf(int line, bool val)
{
  std::cout << "Line: " << line << std::endl;
  return val;
}

#define IF(x) if( x ? printIf(__LINE__, true) : printIf(__LINE__, false) )

int main()
{
  srand(time(NULL));
  //int val;
  //IF (val = rand())
  IF(int val = rand())
  {
    std::cout << "val = " << val << std::endl;
  }
}

Output:

Line: 18
val = 1

Live Example