Question

I have been learning C++ in school to create small command-line programs.

However, I have only built my projects with IDEs, including VS08 and QtCreator.

I understand the process behind building a project: compile source to object code, then link them into an executable that is platform specific (.exe, .app, etc). I also know most projects also use make to streamline the process of compiling and linking multiple source and header files.

The thing is, although IDEs do all this under the hood, making life very easy, I don’t really know what is really happening, and feel that I need to get accustomed to building projects the “old fashioned way”: from the command line, using the tool chain explicitly.

I know what Makefiles are, but not how to write them.
I know what gcc does, but not how to use it.
I know what the linker does, but not how to use it.

What I am looking for, is either an explanation, or link to a tutorial that explains, the workflow for a C++ project, from first writing the code up to running the produced executable.

I would really like to know the what, how, and why of building C++.

(If it makes any difference, I am running Mac OS X, with gcc 4.0.1 and make 3.81)

Thanks!

Answer 1

Compiling

Let’s say you want to write a simple ‘hello world’ application. You have 3 files, hello.cpp hello-writer.cpp and hello-writer.h, the contents being

// hello-writer.h
void WriteHello(void);

// hello-writer.cpp
#include "hello-writer.h"
#include <stdio>
void WriteHello(void){
    std::cout<<"Hello World"<<std::endl;
}

// hello.cpp
#include "hello-writer.h"
int main(int argc, char ** argv){
    WriteHello();
}

The *.cpp files are converted to object files by g++, using the commands

g++ -c hello.cpp -o hello.o
g++ -c hello-writer.cpp -o hello-writer.o

The -c flag skips the linking for the moment. To link all the modules together requires running

g++ hello.o hello-writer.o -o hello

creating the program hello. If you need to link in any external libraries you add them to this line, eg -lm for the math library. The actual library files would look something like libm.a or libm.so, you ignore the suffix and the ‘lib’ part of the filename when adding the linker flag.

Makefile

To automate the build process you use a makefile, which consists of a series of rules, listing a thing to create and the files needed to create it. For instance, hello.o depends on hello.cpp and hello-writer.h, its rule is

hello.o:hello.cpp hello-writer.h
     g++ -c hello.cpp -o hello.o # This line must begin with a tab.

If you want to read the make manual, it tells you how to use variables and automatic rules to simplify things. You should be able to just write

hello.o:hello.cpp hello-writer.h

and the rule will be created automagically. The full makefile for the hello example is

all:hello
hello:hello.o hello-writer.o
    g++ hello.o hello-writer.o -o hello
hello.o:hello.cpp hello-writer.h
    g++ -c hello.cpp -o hello.o
hello-writer.o:hello-writer.cpp hello-writer.h
    g++ -c hello-writer.cpp -o hello-writer.o

Remember that indented lines must start with tabs. Not that not all rules need an actual file, the all target just says create hello. It is common for this to be the first rule in the makefile, the first being automatically created when you run make.

With all this set up you should then be able to go to a command line and run

$ make
$ ./hello
Hello World

More advanced Makefile stuff

There are also some useful variables that you can define in your makefile, which include

• CXX: c++ compiler
• CXXFLAGS:
  Additional flags to pass to the
  compiler (E.g include directories
  with -I)
• LDFLAGS: Additional flags to
  pass to the linker
• LDLIBS: Libraries
  to link
• CC: c compiler (also used to
  link)
• CPPFLAGS: preprocessor flags

Define variables using =, add to variables using +=.

The default rule to convert a .cpp file to a .o file is

$(CXX) $(CXXFLAGS) $(CPPFLAGS) -c $< -o $@

where $< is the first dependancy and $@ is the output file. Variables are expanded by enclosing them in $(), this rule will be run with the pattern hello.o:hello.cpp

Similarly the default linker rule is

$(CC) $(LDFLAGS) $^ -o $@ $(LDLIBS)

where $^ is all of the prerequisites. This rule will be run with the pattern hello:hello.o hello-writer.o. Note that this uses the c compiler, if you don’t want to override this rule and are using c++ add the library -lstdc++ to LDLIBS with the line

LDLIBS+=-lstdc++

in the makefile.

Finally, if you don’t list the dependancies of a .o file make can find them itself, so a minimal makefile might be

LDFLAGS=-lstdc++
all:hello
hello:hello.o hello-writer.o

Note that this ignores the dependancy of the two files on hello-writer.h, so if the header is modified the program won’t be rebuilt. If you’re interested, check the -MD flag in the gcc docs for how you can automatically generate this dependancy.

Final makefile

A reasonable final makefile would be

// Makefile
CC=gcc
CXX=g++
CXXFLAGS+=-Wall -Wextra -Werror
CXXFLAGS+=-Ipath/to/headers
LDLIBS+=-lstdc++ # You could instead use CC = $(CXX) for the same effect 
                 # (watch out for c code though!)

all:hello                                   # default target
hello:hello.o hello-world.o                 # linker
hello.o:hello.cpp hello-world.h             # compile a module
hello-world.o:hello-world.cpp hello-world.h # compile another module
    $(CXX) $(CXXFLAGS) -c $< -o $@          # command to run (same as the default rule)
                                            # expands to g++ -Wall ... -c hello-world.cpp -o hello-world.o