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(C++) main

The main function is the entry point of your program. Have main catch and report every exception [7].

The C++ Standard [1] states that it has one of the following syntaxes:

 

int main() 
{ 
  // Your code
}

and

int main(int argc, char* argv[]) 
{ 
  // Your code
}

What does this all mean?

  1. 'int main': means that main is a function returning an integer. This integer is an error code given back to the operating system the program has run on.
  2. '(int argc, char* argv[])' means that the function takes two arguments:
    1. 'int argc': argc is an abbreviation of 'argument count'. It is a (positive) integer. This give you the number of parameters the user entered when starting the program, which equals 1 if he didn't enter parameters.
    2. 'char* argv[]': an array of pointers to type char (say: 'character pointer'). In argv, all the arguments are stored that the user entered when starting the program. At index 0, the program's name is stored.

 

When main's closing bracket is reached, the effect is equivalent to [6,8]:

return 0;

You could leave out the arguments of main. Then the correct syntax depends on whether you program in C or C++.

Correct C syntax is:

/* Correct C syntax, not correct C++ syntax */
int main(void)
{
  /* Your C code */
  return 0;
}

Unlike C, where one writes void when a function does not have arguments, in C++, when a function has no arguments, nothing is written between the brackets. See the void for more detailed discussion and references.

 

Correct C++ syntax of a main() that does not use its arguments, is [1]:

//Correct C++ syntax
int main()
{
  //Your C++ code
}

Note that the standard states that the closing bracket of main() must have the same effect of returning zero [6,8]. Therefore, return zero can be omitted, but many people like to keep it in.

Incorrect/non-standard is [1-5] (although with some compilers it might compile):

void main() //INCORRECT!!!
{
  //Your code
}

Below is an example showing all parameters a user entered.

#include <iostream>

int main(int argc, char* argv[])
{
  for(int i=0; i!=argc; ++i)
  {
    std::cout << i << " : " << argv[i] << '\n';
  }
}

This means if you start the program with e.g.

./my_main Hello World`

Your output will be something like:

0 : testMain
1 : Hello
2 : World

Should I use void main() or int main()?

int main() [1-5].

Should I use int main() or int main(void)?

References

  1. C++. International Standard. ISO/IEC 14882. Second edition. Paragraph 3.6.1.2
  2. From http://www.parashift.com/c++-faq-lite/newbie.html\#faq-29.3:\

main() must return int. Not void, not bool, not float. int. Just int, nothing but int, only int. Some compilers accept void main(), but that is non-standard and shouldn't be used. Instead use int main().

  1. Herb Sutter. Exceptional C++. ISBN:0-201-61562-2. Item 21: void main() is nonstandard and nonportable.
  2. From Bjarne Stroustrup's homepage (http://www.research.att.com/\~bs/bs\_faq2.html\#void-main):\

The definition

void main() { /* ... */ }

is not and never has been C++, nor has it even been C.

  1. From the The alt.comp.lang.learn.c-c++ FAQ: http://ma.rtij.nl/acllc-c++.FAQ.html\#q3.4: 3.4: Why does everyone make so much fuss about "void main()"?\

Because the return type of the main() function must be int in both C and C++. Anything else is undefined.

Bottom line - don't try to start a thread about this in alt.comp.lang.learn.c-c++ as it has already been discussed many, many times and generates more flamage than any other topic.  

  1. C++. International Standard. ISO/IEC 14882. Second edition. Paragraph 3.6.1.5
  2. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 13.7. Advice. page 387: '[27] 'Have main() catch and report every exception'
  3. Working Draft, Standard for Programming Language C++. International Standard. ISO/IEC document number N3936. 2014-08-22. Paragraph 3.6.1.5
  4. Working Draft, Standard for Programming Language C++. 2014-08-22. N3936. Paragraph C.1.7. First change. 'In C++, a function declared with an empty parameter list takes no arguments. In C, an empty parameter list means that the number and type of the function arguments are unknown.'