An rvalue reference is a C++ reference type written with && that binds to an rvalue, such as the temporary vector returned by get_next_row. Passing that temporary to an appropriate overload lets a container move its contents instead of copying them. section 2.2.3
The distinction matters when choosing vector insertion operations:
- push_back(const T&) copies a fully formed value into the vector. section 2.2.3
- push_back(T&&) accepts an rvalue and can move it into the vector. section 2.2.3
- A move constructor, written with an rvalue reference parameter, can take ownership of a temporary vector’s elements. section 2.2.3
Building a triangle without copying rows
generate_triangle initializes a triangle with {1}, then appends get_next_row(triangle.back()) for each row. Because get_next_row produces a temporary vector, push_back can move that row rather than copy it; emplace_back offers another in-place construction path. section 2.2.3
Listing 2.4 Moving a temporary
auto generate_triangle(int rows)
{
std::vector<std::vector<int>> triangle{ {1} }; #1
for (int row = 1; row < rows; ++row) #2
{
triangle.push_back(get_next_row(triangle.back()));
}
return triangle;
}
A complete triangle-generation listing shows an rvalue-producing function passed directly to vector::push_back. section 2.2.3
