Function to compare contents of variant fails to compile - c++

Question

In my projects I'm using boost-variant exhaustively. Hence, for my unit tests I need to check the contents of a variant against a certain T with a certain content t.
So I deviced the function cmpVariant for this sole purpose and to remove clutter from my unit tests.
In some cases the type T is not equipped with an operator==, so that the user might pass a function satisfying the EqualityCompare Requirement (https://en.cppreference.com/w/cpp/named_req/EqualityComparable)
Now for some obscure reason the following code fails to compile. It says, that there is no matching function?
Clang 6.0.1 Compiler Error
prog.cc:22:5: error: no matching function for call to 'cmpVariant'
cmpVariant(number, 3.2, lambdaEquiv); // Fails!
^~~~~~~~~~
prog.cc:6:6: note: candidate template ignored: could not match 'function<bool (const type-parameter-0-1 &, const type-parameter-0-1 &)>' against '(lambda at prog.cc:19:24)'
bool cmpVariant(
^
1 error generated.
Does anyone knows why?
Code
#include <iostream>
#include <boost/variant.hpp>
#include <functional>
template<typename V, typename T>
bool cmpVariant(
const V& variant,
const T& t,
const std::function<bool(const T& u, const T& v)>& equiv = [](const T& u, const T& v) {return u == v; })
{
if (variant.type() != typeid(t)) return false;
auto v = boost::get<T>(variant);
return equiv(v, t);
}
int main(int, char**) {
boost::variant<double, int> number{ 3.2 };
cmpVariant(number, 3.2);
auto lambdaEquiv = [](const double& x, const double& y) { return x == y; };
std::function<bool(const double&, const double&)> equiv = lambdaEquiv;
cmpVariant(number, 3.2, equiv); // Works!
cmpVariant(number, 3.2, lambdaEquiv); // Fails!
}

Answer 1

The compiler is not able to match the lambda to the function parameter type. You can fix this by explicitly instantiating the function call:
cmpVariant<boost::variant<double, int>, double>(number, 3.2, equiv);
This is clearly a bit wordy, so here is another possibility changing your function declaration to
template<typename V, typename T, typename Fct = std::function<bool(const T& u, const T& v)>>
bool cmpVariant(
const V& variant,
const T& t,
Fct&& f = [](const T& u, const T& v) {return u == v; })
{ /* Same as before. */ }
which can be called like this
cmpVariant(number, 3.2, equiv); // Type deduction works now.
An improvement suggested by #DanielLangr in the comments is to employ std::equal_to.
template<typename V, typename T, typename Fct = std::equal_to<T>>
bool cmpVariant(
const V& variant,
const T& t,
Fct&& f = std::equal_to<T>{})
{ /* Again, same as before. */ }
One advantage here is to get rid of std::function and its often unnecessary overhead.

Answer 2

The way comparator argument is accepted makes deduction problematic, so you may want to change comparator into template parameter (possibly avoiding construction of heavy std::function object ):
template<typename T> class t_EquilityComparator
{
public: bool operator ()(const T& u, const T& v) const { return u == v; }
};
template<typename V, typename T, typename Comparator = t_EquilityComparator<T>>
bool cmpVariant(
const V& variant,
const T& t,
const Comparator & equiv = Comparator{})
{
if (variant.type() != typeid(t)) return false;
auto v = boost::get<T>(variant);
return equiv(v, t);
}
int main(int, char**) {
boost::variant<double, int> number{ 3.2 };
cmpVariant(number, 3.2);
auto equiv = [](const double& x, const double& y) { return x == y; };
cmpVariant(number, 3.2, equiv); // This line fails to compile! Why?
}
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