Disable temporary binding of Eigen expression to const references

问题

I am trying to write a function that accepts only lvalue Eigen expressions passed via const references. My first idea was to keep only the overload const Eigen::MatrixBase<Derived>& and delete the Eigen::MatrixBase<Derived>&& one. To my surprise, the deleted function was not part of the overload candidate set. So I tried the code below

#include <iostream>
#include <Eigen/Dense>

#define PRINT_MY_NAME std::cout << __PRETTY_FUNCTION__ << '\n'

template<typename Derived>
void f(const Eigen::MatrixBase<Derived>&) // (1)
{
    PRINT_MY_NAME;
}

template<typename Derived>
void f(Eigen::MatrixBase<Derived>&&)      // (2)
{
    PRINT_MY_NAME;
}

int main()
{
    Eigen::MatrixXd A;

    f(A);     // invokes (1)
    f(A + A); // invokes also (1) !!!
}

which outputs (gcc5.2)

void f(const Eigen::MatrixBase&) [with Derived = Eigen::Matrix < double, -1, -1>]

void f(const Eigen::MatrixBase&) [with Derived = Eigen::CwiseBinaryOp < Eigen::internal::scalar_sum_op < double>, const Eigen::Matrix < double, -1, -1>, const Eigen::Matrix < double, -1, -1> >]

so clearly the rvalue overload is not considered. It is now clear for me that the second one is not a better match, since I pass a rvalue Eigen expression, which is convertible to Eigen::MatrixBase<>, but is not the exact same type. Now comes my question:

• How can I disable or detect rvalue Eigen expressions passed as parameters for f? The issue is that an expression can have an arbitrary type (Eigen uses expression templates), like CwiseBinaryOp<...CwiseBinaryOp<...>> and so on. This is part of a larger problem in which I have a utility make-like function which takes a lvalue and binds it to a const reference expression in a class. If the expression is a rvalue, then all bets are off, since reference binding is not propagating via a constructor argument, so I want to forbid passing rvalue Eigen expressions.

回答1:

I think I found what was going on: the result of the expression template A + A was a const, so there was a CV-mismatch. Adding const to the second overload does it:

template<typename Derived>
void f(const Eigen::MatrixBase<Derived>&&)      // (2)
{
    PRINT_MY_NAME;
}

来源：https://stackoverflow.com/questions/33201938/disable-temporary-binding-of-eigen-expression-to-const-references