Coding #102: A C++ Project Composition

Hello everyone! I really hope you are enjoying your free time!

In case you are feeling bored, feel free to deep dive into this new episode of Learn2Code.

In Coding #101 we got acquainted with C++, the principal programming language in which Bitcoin Core is written. Today we will go through some of the key components of every C++ projects.

Let’s dive in!

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Header files

A header file is a type of file that stores predefined functions; it allows the programmer to re-use the same functionalities in different parts of the program, thus reducing the complexity and the size of the project.

In C++, header files commonly present a .h extension (or .hpp). In order to add them in a project, the preprocessor directive¹ #include must be used. When a header file is included in a project, all the functionalities (functions, classes, structures) that were decleared in it become available to the programmer.

There are two different kinds of header file:

• Standard library header files: These headers are files that are already present in the C++ compiler and contain functions belonging to the standard library.

  #include <iostream>

• User-defined header files: These are user-defined files that contain previously written code:

  #include <base58.h> // Example taken from Bitcoin Core 

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Source files

While header files contain the definitions of the different functionalities, source files contain their implementation, which means the actual source code.

We can easily identify a C++ source file from its .cpp extension.

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An example from Bitcoin code

To better understand how the two type of files go together, let’s take a look at an example directly from Bitcoin code (I am not going to explain the code yet)

In the base58.h file, it is possible to find the definition of a certain function, DecodeBase58:

bool DecodeBase58(const std::string& str, 
     std::vector<unsigned char>& vchRet,
     int max_ret_len);

At the moment, there is still no information about how this function is implemented; it is possible to find it out by looking at the source file, base58.cpp:

// Include the corresponding header file first
#include <base58.h>

// Implement DecodeBase58 function
bool DecodeBase58(const std::string& str, std::vector<unsigned char>& vchRet, int max_ret_len)
{
    if (!ContainsNoNUL(str)) {
        return false;
    }
    return DecodeBase58(str.c_str(), vchRet, max_ret_len);
}

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1. Preprocessor directive