Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Introduction

[Note] Note

The first 3 chapters are the adapted from the article A Brief Introduction to Rvalue References by Howard E. Hinnant, Bjarne Stroustrup, and Bronek Kozicki

Copying can be expensive. For example, for vectors v2=v1 typically involves a function call, a memory allocation, and a loop. This is of course acceptable where we actually need two copies of a vector, but in many cases, we don't: We often copy a vector from one place to another, just to proceed to overwrite the old copy. Consider:

template <class T> void swap(T& a, T& b)
{
   T tmp(a);   // now we have two copies of a
   a = b;      // now we have two copies of b
   b = tmp;    // now we have two copies of tmp (aka a)
}

But, we didn't want to have any copies of a or b, we just wanted to swap them. Let's try again:

template <class T> void swap(T& a, T& b)
{
   T tmp(::boost::move(a));
   a = ::boost::move(b);
   b = ::boost::move(tmp);
}

This move() gives its target the value of its argument, but is not obliged to preserve the value of its source. So, for a vector, move() could reasonably be expected to leave its argument as a zero-capacity vector to avoid having to copy all the elements. In other words, move is a potentially destructive copy.

In this particular case, we could have optimized swap by a specialization. However, we can't specialize every function that copies a large object just before it deletes or overwrites it. That would be unmanageable.

In C++0x, move semantics are implemented with the introduction of rvalue references. They allow us to implement move() without verbosity or runtime overhead. Boost.Move is a library that offers tools to implement those move semantics not only in compilers with rvalue references but also in compilers conforming to C++03.


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