错误：地图中的operator =含糊不清的重载 <int, std::basic_string<char> &gt; :: Elem :: t3 = 0

Question

I am finishing up work on my second template class (coverting an older map class I made into a template) and I am having the damnedest time figuring out how to initialize null values for a my structure.

I implemented a map using a 2-3-4 tree, where every node is actually a 4-node with the proper values set to NULL (eg a 3-node will have 2 key/value pairs and three children, the the third pair and the four child pointer are set to null.

The issue I am having is that my template throws an error (copied directly from my terminal):

mymap.cpp:271:3: error: ambiguous overload for âoperator=â in âtemp->Map<int, std::basic_string<char> >::Elem::t3 = 0â

The code I am using that is causing this issue is:

//k3 is a KEY value and t3 is a TYPE value
temp -> k3 = NULL;
temp -> t3 = NULL;

Now, I know the issue is that the NULL keyword is normally reserved for pointers, and resolves to \\0 or 0 I believe, and KEY and TYPE could be literally anything, which might not support such values.

My question is what is the proper way to set a null value in such a case?

Thanks!

Answer 1

Often what you want is the value obtained by the default constructor.

#include <utility>
#include <iostream>
#include <string>

template <class A, class B>
void zero(std::pair<A, B>& p)
{
    p.first = A();
    p.second = B();
}

int
main()
{
    std::pair<int, std::string> p { 10, "foo" };

    std::cout << p.first << ' ' << p.second << '\n';
    zero(p);
    std::cout << p.first << ' ' << p.second << '\n';

    return 0;
}

Although, if you are trying to be really general the requirement of a default constructor can be a problem. In that case you usually give the caller some way to specify the default value. Such as:

template <class A, class B>
void zero(std::pair<A, B>& p, A a = A(), B b = B())
{
    p.first = a;
    p.second = b;
}