C++ Primer Plus

The extension you use depends on the C++ implementation. Table 1.1 shows some common choices. For example, spiffy.C is a valid Unix C++ source code filename. Note that Unix is case sensitive, meaning you should use an uppercase C character. Actually, a lowercase c extension also works, but standard C uses that extension. So to avoid confusion on Unix systems, you should use c with C programs and C with C++ programs. If you don’t mind typing an extra character or two, you can also use the cc and cxx extensions with some Unix systems. DOS, being a bit simple-minded compared to Unix, doesn’t distinguish between uppercase and lowercase, so DOS implementations use additional letters, as shown in Table 1.1, to distinguish between C and C++ programs.

Table 1.1. Source Code Extensions

Compilation and Linking

Originally, Stroustrup implemented C++ with a C++-to-C compiler program instead of developing a direct C++-to-object code compiler. This program, called cfront (for C front end), translated C++ source code to C source code, which could then be compiled by a standard C compiler. This approach simplified introducing C++ to the C community. Other implementations have used this approach to bring C++ to other platforms. As C++ has developed and grown in popularity, more and more implementers have turned to creating C++ compilers that generate object code directly from C++ source code. This direct approach speeds up the compilation process and emphasizes that C++ is a separate, if similar, language.

The mechanics of compiling depend on the implementation, and the following sections outline a few common forms. These sections outline the basic steps, but they are no substitute for consulting the documentation for your system.

Unix Compiling and Linking

Originally, the Unix CC command invoked cfront. However, cfront didn’t keep pace with the evolution of C++, and its last release was in 1993. These days a Unix computer instead might have no compiler, a proprietary compiler, or a third-party compiler, perhaps commercial, perhaps freeware, such as the GNU g++ compiler. In many of these other cases (but not in the no-compiler case!), the CC command still works, with the actual compiler being invoked differing from system to system. For simplicity, we’ll assume that CC is available, but realize that you might have to substitute a different command for CC in the following discussion.

You use the CC command to compile your program. The name is in uppercase letters to distinguish it from the standard Unix C compiler cc. The CC compiler is a command-line compiler, meaning you type compilation commands on the Unix command line.

For example, to compile the C++ source code file spiffy.C, you would type this command at the Unix prompt:

CC spiffy.C

If, through skill, dedication, or luck, your program has no errors, the compiler generates an object code file with an o extension. In this case, the compiler produces a file named spiffy.o.

Next, the compiler automatically passes the object code file to the system linker, a program that combines your code with library code to produce the executable file. By default, the executable file is called a.out. If you used just one source file, the linker also deletes the spiffy.o file because it’s no longer needed. To run the program, you just type the name of the executable file:

a.out

Note that if you compile a new program, the new a.out executable file replaces the previous a.out. (That’s because executable files take a lot of space, so overwriting old executable files helps reduce storage demands.) But if you develop an executable program you want to keep, you just use the Unix mv command to change the name of the executable file.

In C++, as in C, you can spread a program over more than one file. (Many of the programs in this book in Chapters 8 through 16 do this.) In such a case, you can compile a program by listing all the files on the command line, like this:

CC my.C precious.C

If there are multiple source code files, the compiler does not delete the object code files. That way, if you just change the my.C file, you can recompile the program with this command:

CC my.C precious.o

This recompiles the my.C file and links it with the previously compiled precious.o file.

You might have to identify some libraries explicitly. For example, to access functions defined in the math library, you may have to add the -lm flag to the command line:

CC usingmath.C -lm

Linux Compiling and Linking

Linux systems most commonly use g++, the GNU C++ compiler from the Free Software Foundation. The compiler is included in most Linux distributions, but it may not always be installed. The g++ compiler works much like the standard Unix compiler. For example, the following produces an executable file call a.out:

g++ spiffy.cxx

Some versions might require that you link in the C++ library:

g++ spiffy.cxx -lg++