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This chapter is from the book

This chapter is from the book

3.3 Modules (C++20)

The use of #includes is a very old, error-prone, and rather expensive way of composing programs out of parts. If you #include header.h in 101 translation units, the text of header.h will be processed by the compiler 101 times. If you #include header1.h before header2.h the declarations and macros in header1.h might affect the meaning of the code in header2.h. If instead you #include header2.h before header1.h, it is header2.h that might affect the code in header1.h. Obviously, this is not ideal, and in fact it has been a major source of cost and bugs since 1972 when this mechanism was first introduced into C.

We are finally about to get a better way of expressing physical modules in C++. The language feature, called modules is not yet ISO C++, but it is an ISO Technical Specification [ModulesTS]. Implementations are in use, so I risk recommending it here even though details are likely to change and it may be years before everybody can use it in production code. Old code, in this case code using #include, can “live” for a very long time because it can be costly and time consuming to update.

Consider how to express the Vector and use() example from §3.2 using modules:

// file Vector.cpp:

module;  // this compilation will define a module

// ... here we put stuff that Vector might need for its implementation ...

export module Vector;  // defining the module called "Vector"

export class Vector {
     Vector(int s);
     double& operator[](int i);
     int size();
     double* elem;     // elem points to an array of sz doubles
     int sz;

Vector::Vector(int s)
     :elem{new double[s]}, sz{s}        // initialize members

double& Vector::operator[](int i)
     return elem[i];

int Vector::size()
     return sz;

export int size(const Vector& v) { return v.size(); }

This defines a module called Vector, which exports the class Vector, all its member functions, and the non-member function size().

The way we use this module is to import it where we need it. For example:

// file user.cpp:

import Vector;         // get Vector's interface
#include <cmath>       // get the standard-library math function interface including sqrt()

double sqrt_sum(Vector& v)
     double sum = 0;
     for (int i=0; i!=v.size(); ++i)
           sum+=std::sqrt(v[i]);        // sum of square roots
     return sum;

I could have imported the standard library mathematical functions also, but I used the old-fashioned #include just to show that you can mix old and new. Such mixing is essential for gradually upgrading older code from using #include to import.

The differences between headers and modules are not just syntactic.

  • A module is compiled once only (rather than in each translation unit in which it is used).

  • Two modules can be imported in either order without changing their meaning.

  • If you import something into a module, users of your module do not implicitly gain access to (and are not bothered by) what you imported: import is not transitive.

The effects on maintainability and compile-time performance can be spectacular.

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