.cpp file must have a declaration for every function that will be used in it.Manually declaring each function from a library (like we did with doubleValue in main.cpp) so that you can use it in another file works, but is a bit cumbersome. Imagine that I want to use a dozen different functions from a library - I would have to start my file with a dozen declarations for the various functions.
library.cpp:
library.h// If we have not yet defined LIBRARY_H, keep going
// Otherwise, skip to the #endif
#ifndef LIBRARY_H
// Define LIBRARY_H so if we end up back in this file we skip over everything
#define LIBRARY_H
// Declare the functions we will be using
/**
* @brief Doubles the value of the input number
*
* @param num an integer value
* @return int 2x the input number
*/
int doubleValue(int num);
#endif
.cpp file, but with a .h extension. This is a convention, not a strict requirement.getMultiplier function is not declared in the header file because it is not intended for use outside of the library.cpp file.library.cpp and how to use it - ideally they should never need to look in the .cpp file.#ifndef ... #endif construct (lines 3, 5, 16). These lines ensure that even if we include this file multiple times, it will only be read one time. Line 3 asks “Have we not defined LIBRARY_H?” (ifndef). If we have not, we keep going. Otherwise, all the code until the endif is skipped. The first thing we do inside the ifndef is to define LIBRARY_H. That way, if we end up in this file again, it will be clear we have already seen the code.
LIBRARY_H).
library.cpp that are not declared in library.h. Doing so indicates that the unlisted functions are not for use by other .cpp files - they are internal functions that other code should not rely on.
string, we need to #include <string> at the top of the header file. However, it is bad practice to use using namespace std; in a header file. The header file code may get included into multiple different .cpp files, and putting using namespace std; in the header means that it will be added to each of those files. Although that will likely work fine in a small project, it could lead to naming conflicts in larger ones. So instead, we should use the full formal name std::string instead of the using namespace shortcut:
otherLibrary.h// If we have not yet defined OTHERLIBRARY_H, keep going
// Otherwise, skip to the #endif
#ifndef OTHERLIBRARY_H
// Define OTHERLIBRARY_H so if we end up back in this file we skip over everything
#define OTHERLIBRARY_H
// Include necessary headers
#include <string>
// using namespace std; <-- Intentionally NOT used here
// We do NOT want to use "using namespace std;" in header files
// because it can lead to name conflicts in larger projects.
// Function just for demo purposes...
// We must use std::string because we are not using "using namespace std;"
std::string firstHalf(const std::string& text);
#endif
mycode.h for a library that just has a void function called print that takes a string parameter. You will not use all of the blocks.
