Evgenij LegotskojMay 7, 2017, 4:51 a.m.

C++ - Tutorial 001. Hello World

The minimum program in C ++ is

int main() { } // the minimal C++ program

In this program, the declaration of the main function is presented, which takes no arguments. The curly brackets reflect grouping in C++ and in this case show the body of the main function. That is, the beginning of the main function is the opening parenthesis, and the end of the main function is the closing parenthesis. A double slash shows the beginning of the comment. Comments are ignored by the compiler and serve to refine the information in the code.

Each program written in C++ has the function main() , with which the program starts. The main() function, as a rule, returns the result of its execution, which is indicated by the int (integer) data type written before the main() function. If the function is successful, the main() function returns 0 as the result. The value of the result, other than zero, signals an abnormal termination of the program.

The value returned by the program upon completion can be used in the operating system for service purposes.

A typical example of the first program in any programming language is the output of the text "Hello, World!" :

#include <iostream>

int main()
{
    std::cout << "Hello, World!\n";
}

But is it so simple in this program? In general, already this one small program carries a very large layer of information, which must be understood for development in C++.

  1. #include directive
    #include
    tells the compiler that it is necessary to connect a header file, the components of which are planned to be used in the file where the main() function is declared. iostream is a standard input library from STL. That is, the functional of libraries is already used here, although it is the standard for the language. And the last point is the angle brackets, in which the name of the library is located, which say that this is the inclusion of external files in the project, and not those that are part of the project. The same files that are included in the project are connected by enclosing them in normal quotes, for example #include "myclass.h" . This library connection is standard. For example, in Visual Studio , if this standard is not followed, errors will occur.
  2. std is the use of the namespace in which the cout output statement is located. Namespaces were introduced in C ++ in order to remove name conflicts between libraries and the developer's project, if there are repeated names of functions or classes somewhere. In Java, the package system is used to resolve name conflicts.
  3. cout is an output operator that has an overloaded operator << , in order not to use a separate function to output text to the console.

This is in addition to the fact that the record of the main function can have a different form, although the standard is two records:

  1. int main()
  2. int main(int argc, char* argv[])

You can find more records like void main() , etc. But these are erroneous records, although in some compilers they will be compiled, even without errors and warnings.

In the record int main (int argc, char * argv []) arguments are passed:

  1. argc - indicates the number of arguments passed. Always at least 1, because the name of the program is always passed
  2. argv[] - an array of pointers to arguments that are passed as string variables.

If argc is greater than 1, then additional arguments were passed when the program was started.

The check can look like this:

#include <iostream>

int main(int argc, char* argv[])
{
    // If an additional argument is passed,
    if (argc > 1)
    {
        // Then try to derive the argument
        std::cout << argv[1]<<endl;
    } 
    else
    {
        // Otherwise, we inform you that the arguments were not transmitted
        cout << "Without arguments" << endl;
    }
    return 0;
}

In general, there are a lot of things that you need to understand in C ++, even for a small program, but this is only more interesting ;-)

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Забыли std
std::cout << argv[1]<< std:: endl; //строка 9
std:: cout << "Without arguments" << std:: endl; //строка 14

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