16.3) Library Header File

Suppose we wish to distribute the three functions from the statistical analysis program as a single library. We can create the following header file called data.hpp for this library:

1    #ifndef DATA_HPP
2    #define DATA_HPP
3
4    #include <iostream>
5    #include <vector>
6
7    void read_data(std::istream&, std::vector<double>&);
8    double mean(const std::vector<double>&);
9    double stdev(const std::vector<double>&);
10
11   #endif

• Lines 1, 2 & 11 are an ‘include guard’ - ensuring that lines in between will be seen the first time that this header file is included and ignored on any subsequent includes; an alternative approach, supported by most compilers, is to use #pragma once at the start of the file
• Lines 4 & 5 ensure that the compiler has seen definitions of istream and vector, which is necessary for the prototypes on lines 7–9 make sense
• Note the use of std:: here; we don’t want to put a using namespace std in the header file because it would impose that decision on every user of the library