为什么 main 函数输出 73.4 即使它的返回类型是 int？

Question

I am a beginner to C++ and am confused with the usage of int main(); . I have looked at some threads on this on the website, but I will be glad if someone could provide insight into my problem.

For the following program, if my input is 23, the output value is 73.4, which is NOT an int value contrary to the fact that my definition of the main() function should return an int value only.

What am I missing here? Thank you!

My program is as follows:

#include <iostream>
using namespace std;

int main(){

  double Celsius, Farenheit; // defining variables celsius and farenheit
  cout << " Input temperatue in celsius and press ENTER" << endl;
  cin >> Celsius;

  //double Farenheit;
  Farenheit = ((Celsius*180)/100) + 32;
  cout << "Value in Farenheit is:";
  cout << Farenheit <<endl ;

  //system("Pause");
  return 0;

  }

Answer 1

Indeed, the output of your program is 73.4 , but output and return values are a different things.

A program can write in the console, in fact any function can write to the console. But the return value of a function is different than the output it produced.

In your code, the return value of the main function is not 73.4 . But that's the output it produced in the console. It's return value is what's following the return statement:

return 0;

Yep! That's right! The return value is what value is placed after the return keyword!

Here's another example of a function returning an int and producing an output to the console:

int calculate() {
    std::cout << "calculating stuff..." << std::endl;
    return 2 * 2 * 4;
}

int main() {
    int return_value = calculate();
    std::cout << "return value of calculate() is: "
              << return_value << std::endl;

    return 0;
}

This produces the output:

calculating stuff...

return value of calculate() is: 16

Now maybe you wonder why return 0 from main . This is usually to indicate to the operating system that the program has executed successfully.

Answer 2

The purpose of main 's return value is to return an exit status to the operating system. In standard C, the only valid signatures for main are: int main(void) and int main(int argc, char **argv) The form you're using: int main() is an old style declaration that indicates maintakes an unspecified number of arguments

to solve your problem. change the data types.aa

int celcius,fahrenheit;

or

double celcius; int fahrenheit;

Answer 3

Your definition of Farenheit makes it a variable of type double , and therefore it would store a value of type double . The int of int main() have nothing to do with this. It represents the type of the return type of main() , which is 0 because of your last line of the program, return 0 . In fact, you can actually consider int in int main() completely useless for now.

To make the program output an integer, you should define Farenheit as an int

#include <iostream>
using namespace std;

int main(){

  double Celsius;
  int Farenheit; // defining variables celsius and farenheit
  cout << " Input temperatue in celsius and press ENTER" << endl;
  cin >> Celsius;

  //double Farenheit;
  Farenheit = ((Celsius*180)/100) + 32;
  cout << "Value in Farenheit is:";
  cout << Farenheit <<endl ;

  //system("Pause");
  return 0;

  }