ambiguity of overloaded function taking constant Eigen argument - c++

Question

I've designed a class with two overloaded functions taking Eigen data structures of different sizes.
The code compiles as long as I'm passing lvalues but if I pass an rvalue I get a compiler error ambiguity because both return the same ConstantReturnType.
Here is a MWE:
#include <iostream>
#include <Eigen/Geometry>
using namespace std;
using namespace Eigen;
class MyOverloadAmbiguity {
public:
void ambiguousOverload(const Eigen::Vector3d& v) {
std::cout << "I'm taking a Vector3d\n";
}
void ambiguousOverload(const Eigen::Vector4d& v){
std::cout << "I'm taking a Vector4d\n";
}
};
int main()
{
MyOverloadAmbiguity moa;
Eigen::Vector3d v3;
moa.ambiguousOverload(v3); // <--- this works
moa.ambiguousOverload(Eigen::Vector4d::Zero()); // <--- this doesn't
return 0;
}
main.cpp:26: error: call of overloaded ‘ambiguousOverload(const ConstantReturnType)’ is ambiguous
26 | moa.ambiguousOverload(Eigen::Vector4d::Zero());
| ^
main.cpp:10:8: note: candidate: ‘void MyOverloadAmbiguity::ambiguousOverload(const Vector3d&)’
10 | void ambiguousOverload(const Eigen::Vector3d& v) {
| ^~~~~~~~~~~~~~~~~
main.cpp:13:8: note: candidate: ‘void MyOverloadAmbiguity::ambiguousOverload(const Vector4d&)’
13 | void ambiguousOverload(const Eigen::Vector4d& v){
| ^~~~~~~~~~~~~~~~~
Is there a way to avoid this without explicitly changing the function names or add extra arguments just to avoid the ambiguity?

Answer 1

Your example does not work because the return type of Zero() is not a matrix, but an Eigen expression.
Thus, one way of achieving what you want with minimal changes is to use explicit matrix evaluation:
moa.ambiguousOverload(Eigen::Vector4d::Zero().eval());
You may also want to consider writing functions taking Eigen expressions as parameters (rather than explicit matrices), as an alternative solution.