c++ - Eigen 中的 move 语义

Question

我有几个关于 Eigen 的问题:

1. 有谁知道近期是否有计划在 Eigen 中支持 move 语义？在 Eigen3 网页的 TODO 列表中找不到任何内容。现在我正在使用 swap 技巧来摆脱临时对象，比如

   MatrixXd foo()
   {
       MatrixXd huge_matrix(N,N); // size N x N where N is quite large
       // do something here with huge_matrix
       return huge_matrix; 
   }
   
   MatrixXd A(N, N); 
   A.swap(foo());
   

   我非常想用 C++11 风格编写上面的 swap 行

   A = foo();
   

   并且不必担心 foo() 返回的临时值。

2. C++98/C++03 编译器可以优化代码 A = foo(); 来摆脱这个临时问题吗？或者最安全的选择是使用 swap()？

Answer 1

Copy elision会做的很好。即使是 C++03 编译器也会省略临时文件。
特别是，对于此代码，NRVO(命名返回值优化)将

MatrixXd foo()
{
    MatrixXd huge_matrix(N,N);
    return huge_matrix; 
}

MatrixXd A = foo();

在 A 内部构造 huge_matrix。 C++03标准在[class.copy]/15中规定:

This elision of copy operations is permitted in the following circumstances (which may be combined to eliminate multiple copies):

• in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object with the same cv-unqualified type as the function return type, the copy operation can be omitted by constructing the automatic object directly into the function’s return value
• when a temporary class object that has not been bound to a reference (12.2) would be copied to a class object with the same cv-unqualified type, the copy operation can be omitted by constructing the temporary object directly into the target of the omitted copy