Member Variable Pointer
-
21-12-2019 - |
Question
For a given struct:
struct foo
{
void fooFunc(){}
int fooVar = 0;
};
I can create a call wapper to the function: std::mem_fn( &foo::fooFunc )
such that I can pass it into another method and call it on an object.
I want to know if there is a similar call wrapper but for member variables.
For example I'm using a pointer to a member variable here and but I'd like to use an call wrapper:
void bar( std::function< void( foo ) > funcPtr, int foo::* varPtr )
{
foo myFoo;
funcPtr( myFoo );
foo.*varPtr = 13;
}
Solution
N.B. Nothing in your question is from the STL (which is a library from the 1990s), std::function
and std::mem_fn
are part of the C++ Standard Library, which is not the same thing.
std::mem_fn
supports member functions and member variables, so you can just do this to access the member variable and set it:
foo f;
std::function<int&(foo&)> var( std::mem_fn( &foo::fooVar ) );
var(f) = 1;
OTHER TIPS
In addition to member functions, std::mem_fn
can wrap data members too, and allow read access to them. So the following works:
void print_fooVar(foo& f, std::function<int(foo)> varAccess)
{
std::cout << varAccess(f) << std::endl;
}
foo f;
print_fooVar(f, std::mem_fn(&foo::fooVar)); // prints 0
As JonathanWakely mentions in the comments, you can use the (unspecified) type returned by mem_fn
itself to set the data member.
foo f;
std::mem_fn(&foo::fooVar)(f) = 13;
Or to transform it into an std::function
use
void bar( foo& f, std::function<int&(foo&)> fooVarSet )
{
fooVarSet(f) = 26;
}
If all you're looking for is a way to generate a callable to set the fooVar
data member, and not using std::mem_fn
specifically to do this, you can get the job done using a lambda expression as well.
void bar( foo& f, std::function<void(foo)> funcPtr,
std::function<void(foo&, int)> fooVarSet )
{
funcPtr( f );
fooVarSet(f, 13);
}
foo f;
bar(f, std::mem_fn(&foo::fooFunc), [](foo& f, int i) {f.fooVar = i;});