I have always seen people write
class.h
#ifndef CLASS_H
#define CLASS_H
//blah blah blah
#endif
The question is, why don't they also do that for the .cpp file that contain definitions for class functions?
Let's say I have main.cpp
, and main.cpp
includes class.h
. The class.h
file does not include
anything, so how does main.cpp
know what is in the class.cpp
?
Use “why” in a sentenceWhy don't we go swimming this afternoon? Why don't you try a taste of this? I can't figure out why he didn't tell the truth. Could you please tell me again why you are late?
The conjunctions and and or connote very different meanings. In particular, and suggests the joint consideration of two concepts, whereas or suggests exclusivity.
For: purpose We use for to talk about a purpose or a reason for something: I'm going for some breakfast. I'm really hungry. She leaves on Friday for a 15-day cruise around the Mediterranean.
To is a preposition and a versatile little word that can be used to say many things. You can use it to indicate a goal or a direction of movement, as well as a place of arrival. That's the way you use it when you say you're going to class tomorrow.
First, to address your first inquiry:
When you see this in .h file:
#ifndef FILE_H
#define FILE_H
/* ... Declarations etc here ... */
#endif
This is a preprocessor technique of preventing a header file from being included multiple times, which can be problematic for various reasons. During compilation of your project, each .cpp file (usually) is compiled. In simple terms, this means the compiler will take your .cpp file, open any files #included
by it, concatenate them all into one massive text file, and then perform syntax analysis and finally it will convert it to some intermediate code, optimize/perform other tasks, and finally generate the assembly output for the target architecture. Because of this, if a file is #included
multiple times under one .cpp file, the compiler will append its file contents twice, so if there are definitions within that file, you will get a compiler error telling you that you redefined a variable. When the file is processed by the preprocessor step in the compilation process, the first time its contents are reached the first two lines will check if FILE_H
has been defined for the preprocessor. If not, it will define FILE_H
and continue processing the code between it and the #endif
directive. The next time that file's contents are seen by the preprocessor, the check against FILE_H
will be false, so it will immediately scan down to the #endif
and continue after it. This prevents redefinition errors.
And to address your second concern:
In C++ programming as a general practice we separate development into two file types. One is with an extension of .h and we call this a "header file." They usually provide a declaration of functions, classes, structs, global variables, typedefs, preprocessing macros and definitions, etc. Basically, they just provide you with information about your code. Then we have the .cpp extension which we call a "code file." This will provide definitions for those functions, class members, any struct members that need definitions, global variables, etc. So the .h file declares code, and the .cpp file implements that declaration. For this reason, we generally during compilation compile each .cpp file into an object and then link those objects (because you almost never see one .cpp file include another .cpp file).
How these externals are resolved is a job for the linker. When your compiler processes main.cpp, it gets declarations for the code in class.cpp by including class.h. It only needs to know what these functions or variables look like (which is what a declaration gives you). So it compiles your main.cpp file into some object file (call it main.obj). Similarly, class.cpp is compiled into a class.obj file. To produce the final executable, a linker is invoked to link those two object files together. For any unresolved external variables or functions, the compiler will place a stub where the access happens. The linker will then take this stub and look for the code or variable in another listed object file, and if it's found, it combines the code from the two object files into an output file and replaces the stub with the final location of the function or variable. This way, your code in main.cpp can call functions and use variables in class.cpp IF AND ONLY IF THEY ARE DECLARED IN class.h.
I hope this was helpful.
The CLASS_H
is an include guard; it's used to avoid the same header file being included multiple times (via different routes) within the same CPP file (or, more accurately, the same translation unit), which would lead to multiple-definition errors.
Include guards aren't needed on CPP files because, by definition, the contents of the CPP file are only read once.
You seem to have interpreted the include guards as having the same function as import
statements in other languages (such as Java); that's not the case, however. The #include
itself is roughly equivalent to the import
in other languages.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With