How to use  g++


GNU provides are a publicly-available optimizing compilers (translator) for C, C++, Ada 95, and Objective C that currently runs under various implementations of Unix (plus VMS as well as OS/2 and perhaps other PC systems) on a variety of processors too numerous to mention. You can find full documentation on-line under Emacs (use C-h i and select the ``GCC'' menu option). You don't need to know much about it for our purposes. This document is a brief summary. (Note that g++ and gcc are, for most practical purposes, identical programs. Running g++ is nearly the same as gcc -lg++. Consequently, when looking at GNU's online documentation using Emacs info, gcc is where you'll look.)  Also, you can try man g++.

Running the compiler

You can use g++ both to compile programs into object modules and to link these object modules together into a single program. It looks at the names of the files you give it to determine what language they are in and what to do with them. Files of the form (or name.cpp) are assumed to be C++ files and files matching name.o are assumed to be object (i.e., machine-language) files. To translate a C++ source file,, into a corresponding object file, file.o, use the g++ command:
  g++ -c compile-options

To link one or more object files, file1.o, file2.o, ...,
to produced from C++ files into a single executable file called prog,
use the following command:
  g++ -o prog link-options file1.o file2.o ... other-libraries
(The options and libraries clauses are described below.)

You can bunch these two steps---compilation and linking---into one with the following command.

  g++ -o prog compile-and-link-options ... other-libraries
examples: g++ -o test1 test1.cpp
          g++ -ansi -o prog2 fig02_07.cpp
          g++ -g -o prog3 fig02_09.cpp
          g++ -Wall -g prog4 fig02_21.cpp -lm    
After linking has produced an executable file called prog, it becomes, in effect, a new Unix command, which you can run with
  ./prog arguments
where arguments denotes any command-line arguments to the program.


A library is a collection of object files that has been grouped together into a single file and indexed. When the linking command encounters a library in its list of object files to link, it looks to see if preceding object files contained calls to functions not yet defined that are defined in one of the library's object files. When it finds such a function, it then links in the appropriate object file from the library. One library gets added to the list of libraries automatically, and provides a number of standard functions common to C++ and C.

Libraries are usually designated with an argument of the form -llibrary-name. In particular, -lg++ denotes a library of standard C++ routines and -lm denotes a library containing various mathematical routines (sine, cosine, arctan, square root, etc.) They must be listed after the object or source files that contain calls to their functions.


The following compile- and link-options will be of particular interest to us.
-c (Compilation option)
Compile only. Produces .o files from source files without doing any linking.
-Dname=value (Compilation option)
In the program being compiled, define name as if there were a line
  #define name  value
at the beginning of the program. The ` = value' part may be left off, in which case value defaults to 1.
-o file-name (Link option, usually)
Use file-name as the name of the file produced by g++ (usually, this is an executable file).
-llibrary-name (Link option)
Link in the specified library. See above. (Link option).
-g (Compilation and link option)
Put debugging information for gdb into the object or executable file. Should be specified for both compilation and linking.
-MM (Compilation option)
Print the header files (other than standard headers) used by each source file in a format acceptable to make. Don't produce a .o file or an executable.
-pg (Compilation and link option)
Put profiling instructions for generating profiling information for gprof into the object or executable file. Should be specified for both compilation or linking. Profiling is the process of measuring how long various portions of your program take to execute. When you specify -pg, the resulting executable program, when run, will produce a file of statistics. A program called gprof will then produce a listing from that file telling how much time was spent executing each function.
-Wall (Compilation option)
Produce warning messages about a number of things that are legal but dubious. I strongly suggest that you always specify this and that you treat every warning as an error to be fixed.