什么时候需要#include .cpp文件？

Question

One of my homeworks involves three files: LineType.h, LineType.cpp, and Driver.cpp. Driver.cpp contains the main() method which uses a class defined by LineType.h and LineType.cpp.

On my system, Driver.cpp starts with:

#include "LineType.h"
#include "LineType.cpp"
#include <iostream>

And the program compiles and runs perfectly when I run g++ Driver.cpp from within the project directory via the command line. However, when my instructor attempts to compile the program (I believe she uses Eclipse), it fails to compile. After some back-and-forth, she was able to fix the problem on her end by commenting out one of the #includes from Driver.cpp:

#include "LineType.h"
//#include "LineType.cpp"
#include <iostream>

When I attempt to run g++ Driver.cpp on this edited file, my compiler complains about "Undefined symbols for architecture", which I understand to mean that it cannot find definitions for the class/methods being called.

What are my instructor and I doing differently to cause this difference in behavior? Why does a line required by my compiler cause her compiler to fail?

Answer 1

You should never include source files directly.

Instead, you should list all your source files in the g++ command when you compile:

g++ Driver.cpp LineType.cpp MyOtherFile.cpp # etc...

Answer 2

Using #include somefilename means that content of somefilename is put in place of the include.
By putting #include "LineType.cpp" in your Driver.cpp file you efectively put everythig in one file and then compiling using g++ Driver.cpp works fine for you.
When your instructor used IDE for compiling it went on separate compile and linking. So it compiled Driver.cpp and LineType.cpp Both files contain definitions from LineType.cpp due to that include. So when it came to linking, she had everything definded in LineType.cpp twice and linker didn't know what to do. You can compile and link multiple files at once by using

g++ Driver.cpp LineType.cpp 

Or using separate compile and linking commands

g++ -c Driver.cpp
g++ -c LineType.cpp

Which will generate files Driver.o and LineType.o . Then you can combine them together by running

g++ Driver.o LineType.o

Answer 3

Personally I strongly do not recommend to include source files. But author of this article claims that including source files can reduce large project compilation time in orders. He calls this 'unity builds' and claims the approach is widely used in games industry. The main idea of unity build is to reduce the number of modules in a compilation. Like this:

my_unity_build_1.cpp:

#include "renderer.cpp"
#include "ui_elements.cpp"
#include "gameplay_code.cpp"
#include "character_AI.cpp"

my_unity_build_2.cpp:

#include "file_io.cpp"
#include "cat_dynamics.cpp"
#include "wobbly_bits.cpp"
#include "death_ray.cpp"

Fewer modules mean less duplication common functions and less code generation. Modules allow to reduce compilation time dramatically but still are not in standard.