Forward declaration of std::hash

Question

I have multiple classes which overloads std::hash operator(). Let's say:

A.h

class A
{
...
}

namespace std
{
  template<typename T>
  struct hash;
}

A.cpp

template<> struct std::hash<A>
{
  public:
  virtual std::size_t operator()(const A& joint) const noexcept
  {
..

}

And similar classes let's say class B and class C

Now class B uses A's hash like:

boost::hash_combine(h, std::hash<A>{}(b.getA())

Problem: error: use of deleted function std::hash<A>::hash()

I tried: A.h

namespace std
{
  template<typename T>
  struct hash;
}

B.h

class B 
{
   friend struct std::hash<A>;
}

Do you really have `template<> struct std::hash` in `A.cpp`? — NathanOliver, May 08 '20 at 20:51
@tadman specializing `std::hash` is 100% allowed and is idiomatic if you want your object to be useable in the unordered associative containers. — NathanOliver, May 08 '20 at 20:52
Most likely this is a dupe of: https://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file — NathanOliver, May 08 '20 at 20:53
@NathanOliver Yes I have defined `std::hash` in A.cpp like I said. — code muncher, May 08 '20 at 20:55
@NathanOliver the pointer you pointed, does it mean I have to write ALL the definitions of std::hash<> .. for all of my class in one cpp file? I have more than `class C` which depends upon `A` and `B` . — code muncher, May 08 '20 at 20:57
No, put each specialization in each classes header file. Template code for the most part needs to live in header files for the compiler to be able to do its thing. — NathanOliver, May 08 '20 at 20:58
@NathanOliver Correct, that's exactly where I'm not sure. How to just declare the std::hash ? — code muncher, May 08 '20 at 21:04
Move the `template<> struct std::hash ...` from A.cpp to A.h — NathanOliver, May 08 '20 at 21:05
Oh you mean the whole definition I should move to header file? Well then the problem is `B` is using `A`. `C` is using `A` and `B`. I can't really put the definition in headerfile — code muncher, May 08 '20 at 21:07
@NathanOliver: The `std::hash` isn't actually template code though. It doesn't need to be in a header. — Mooing Duck, May 08 '20 at 21:09
@MooingDuck You still have to have it declared in the header file, otherwise dependent code doesn't know there is a specialization and will use the default template. — NathanOliver, May 08 '20 at 21:14

Brian Bi · Accepted Answer · 2020-05-08T21:08:42.000

3

You have to define the std::hash<A> specialization in A.h, so that B and C will be able to see that the specialization exists and that it has an operator() member.

If B and C can't "see" the std::hash<A> specialization, then they will instantiate the primary std::hash template, which is disabled because the standard library doesn't know how to hash your user-defined type A.

If you declare the std::hash<A> specialization in A.h, but you do not define it, then B and C will see it as an incomplete class, which means the operator() cannot be called.

So you must define std::hash<A> in A.h. You can do so like this:

// A.h
namespace std {
template <> struct hash<A> {
    std::size_t operator()(const A&) const noexcept { ... }
};
}

You could also move the definition of operator() to the A.cpp file:

// A.h
namespace std {
template <> struct hash<A> {
    std::size_t operator()(const A&) const noexcept;
};
}

// A.cpp
namespace std {
std::size_t hash<A>::operator()(const A&) const noexcept { ... }
}

edited May 08 '20 at 21:08

answered May 08 '20 at 20:55

Brian Bi

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I tried to just define those function, but had no success. I edited the question to show how I am defining right now – code muncher May 08 '20 at 21:00
@codemuncher As I said, a definition of the `std::hash` specialization needs to be placed **in the header file**, that is, `A.h`. – Brian Bi May 08 '20 at 21:01
Yes agreed. Could you please update answer on how to correctly declare the `std::hash`? I tried multiple ways and I get 1) duplicate definition. 2) The way I defined right now is given in question. – code muncher May 08 '20 at 21:03
@codemuncher: The key insight is to separate the concepts of the declaration of `std::hash`, the defintion of `std::hash `, the declaration of `std::hash ::operator()`, and the definition of `std::hash ::operator()`. All declarations should be in headers. – Mooing Duck May 08 '20 at 21:11
@Brian Looks like I had my Cpp 101 lessons forgotten! I had declared it fine but when I was defining I was redefining the struct. Shit! THANK YOU! – code muncher May 08 '20 at 21:22

Forward declaration of std::hash

1 Answers1