I cannot pass lambda as std::function
Let's focus on this example:
template<typename T>
class C{
public:
void func(std::vector<T>& vec, std::function<T( const std::string)>& f){
//Do Something
}
};
And now, I am trying:
std::vector<int> vec;
auto lambda = [](const std::string& s) { return std::stoi(s); };
C<int> c;
c.func(vec, lambda);
It causes errors:
no matching function for call to ‘C<int>::func(std::vector<int, std::allocator<int> >&, main()::<lambda(const string&)>&)’
ref.parse(vec, lambda);
Please explain me what is not ok and how to implement it with std::bind as well.
Solution 1:
It's because a lambda function is not a std::function<...>
. The type of
auto lambda = [](const std::string& s) { return std::stoi(s); };
is not std::function<int(const std::string&)>
, but something unspecified which can be assigned to a std::function
. Now, when you call your method, the compiler complains that the types don't match, as conversion would mean to create a temporary which cannot bind to a non-const reference.
This is also not specific to lambda functions as the error happens when you pass a normal function. This won't work either:
int f(std::string const&) {return 0;}
int main()
{
std::vector<int> vec;
C<int> c;
c.func(vec, f);
}
You can either assign the lambda to a std::function
std::function<int(const std::string&)> lambda = [](const std::string& s) { return std::stoi(s); };
,change your member-function to take the function by value or const-reference or make the function parameter a template type. This will be slightly more efficient in case you pass a lambda or normal function pointer, but I personally like the expressive std::function
type in the signature.
template<typename T>
class C{
public:
void func(std::vector<T>& vec, std::function<T( const std::string)> f){
//Do Something
}
// or
void func(std::vector<T>& vec, std::function<T( const std::string)> const& f){
//Do Something
}
// or
template<typename F> func(std::vector<T>& vec, F f){
//Do Something
}
};
Solution 2:
It's because the argument (std::function
) is a reference. It should be:
void func(std::vector<T>& vec, std::function<T(const std::string&)> f)
^ ^
|
f not a reference
So that the argument can be converted to the parameter type.
Also, the type of the function should match. I.e. it should accept a string reference.