1.1. Writing a Simple C++ Program

Every C++ program contains one or more functions, one of which must be named main. The operating system runs a C++ program by calling main. Here is a simple version of main that does nothing but return a value to the operating system:

int main()
{
    return 0;
}

A function definition has four elements: a return type, a function name, a (possibly empty) parameter list enclosed in parentheses, and a function body. Although main is special in some ways, we define main the same way we define any other function.

In this example, main has an empty list of parameters (shown by the () with nothing inside). § 6.2.5 (p. 218) will discuss the other parameter types that we can define for main.

The main function is required to have a return type of int, which is a type that represents integers. The int type is a built-in type, which means that it is one of the types the language defines.

The final part of a function definition, the function body, is a block of statements starting with an open curly brace and ending with a close curly:

{
    return 0;
}

Whether you use a command-line interface or an IDE, most compilers expect program source code to be stored in one or more files. Program files are normally referred to as a source files. On most systems, the name of a source file ends with a suffix, which is a period followed by one or more characters. The suffix tells the system that the file is a C++ program. Different compilers use different suffix conventions; the most common include .cc, .cxx, .cpp, .cp, and .C.

Running the Compiler from the Command Line

If we are using a command-line interface, we will typically compile a program in a console window (such as a shell window on a UNIX system or a Command Prompt window on Windows). Assuming that our main program is in a file named prog1.cc, we might compile it by using a command such as

$ CC prog1.cc

where CC names the compiler and $ is the system prompt. The compiler generates an executable file. On a Windows system, that executable file is named prog1.exe. UNIX compilers tend to put their executables in files named a.out.

To run an executable on Windows, we supply the executable file name and can omit the .exe file extension:

$ prog1

On some systems you must specify the file’s location explicitly, even if the file is in the current directory or folder. In such cases, we would write

$ .\prog1

The “.” followed by a backslash indicates that the file is in the current directory.

To run an executable on UNIX, we use the full file name, including the file extension:

$ a.out

If we need to specify the file’s location, we’d use a “.” followed by a forward slash to indicate that our executable is in the current directory:

$ ./a.out

The value returned from main is accessed in a system-dependent manner. On both UNIX and Windows systems, after executing the program, you must issue an appropriate echo command.

On UNIX systems, we obtain the status by writing

$ echo $?

To see the status on a Windows system, we write

$ echo %ERRORLEVEL%