Specialize hashmap template

Question

I'm using the hash_map in C++ and want to supply a simplified type name for it:

The key type and hasher function are always the same.

stdext::hash_map<std::string, [MAPPED TYPE], CStringHasher>

However, I don't really want to write all this every time I declare a hash map which maps strings to type X.

Can I write a template or macro that simplifies this?

So the above declaration would look like:

template<typename T> StringHashmap = stdext::hash_map<std::string, T, CStringHasher>

StringHashmap<int> mapA; //Maps std::string to int
StringHashamp<bool> mapB; //Maps std::string to bool

score 5 · Accepted Answer · edited May 23 '17 at 11:47

5

As others said, template aliases is the way to go if you can use C++0x:

template < typename MappedType >
using StringHashMap = stdext::hash_map< std::string, MappedType, CStringHasher >;

StringHashMap< int > mapA;
StringHashMap< bool > mapB;

(As @MSalters noted, if you have C++0x available, you should probably use std::unordered_map.)

Otherwise, you can still use the usual workaround, which is to define a class template containing a typedef:

template < typename MappedType >
struct StringHashMap
{
    typedef stdext::hash_map< std::string, MappedType, CStringHasher > type;
};

StringHashMap< int >::type mapA;
StringHashMap< bool >::type mapB;

A drawback with this solution (which have engendered many questions here on SO) is that you sometimes need to prefix StringHashMap< T >::type by the typename keyword in order to assert to the compiler that this name effectively denotes a type. I will not dwell on this subject in this answer, you can check out this FAQ, and especially the accepted answer, for more information. (Thanks to @sbi for evoking this matter.)

edited May 23 '17 at 11:47

Community

1
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answered Feb 11 '11 at 09:35

Luc Touraille

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+1 This is a much better answer than mine. I'm going to have to remember this trick. – templatetypedef Feb 11 '11 at 09:44
Note that `StringHashMap::type` needs to be qualified as a type name when used in a template where `T` is (or is dependent from) a template parameter: `typename StringHashMap::type`. There is an [FAQ entry for that](http://stackoverflow.com/questions/610245) on SO. You might want to spend some words on this issue. Nevertheless, `+1` for a good answer. – sbi Feb 11 '11 at 10:01
1

If you had C++0x, you'd be using `std::unordered_map` instead of `stdext::hash_map`. – MSalters Feb 11 '11 at 10:10
@sbi @MSalters: I edited the answer to include your remarks, thanks for the comments. – Luc Touraille Feb 11 '11 at 12:46

templatetypedef · Answer 2 · 2011-02-11T09:48:36.707

With the current version of C++ (C++03), there really isn't a good way to do this. You could try something like

#define StringHashMap(type) stdext::hash_map<std::string, type, CStringHasher>

But then you run into trouble if you try to specify a template type with a comma in it, like this:

StringHashMap(pair<int, string>) myMap; // Error!

This fails because the preprocessor will tokenize this as

StringHashMap((pair<int), (string>)) myMap; // Error!

which isn't what you want. If you're not going to do anything like this, though, you should be fine. You could alternatively use typedefs to get around it:

typedef pair<int, int> IntPair;
StringHashMap(IntPair) myMap; // Okay

If you're allowing use of C++0x features, you can use a template using declaration, like this one here:

template <typename T>
    using StringHashMap = stdext::hash_map<std::string, T, CStringHasher>;

Unfortunately, C++03 doesn't have a "template typedef" feature like this one, so you'd have to use a newer compiler and won't have as good a portability guarantee.

Hey, wait a minute! My name is templatetypedef and this is the first time on Stack Overflow that I've ever written that phrase in an answer! You just made my night. :-)

+1 for having the appropriate user-name to be answering this question. — Scott Langham, Feb 11 '11 at 09:33
I think you need to remove the `` after `StringHashMap` in the using declaration. Good remarks about the issue of commas in macro arguments. — Luc Touraille, Feb 11 '11 at 09:46

score 1 · Answer 3 · answered Feb 11 '11 at 09:30

1

If you're using C++ 0x, template typedefs were designed to do exactly this kind of thing:

http://en.wikipedia.org/wiki/C%2B%2B0x#Template_aliases

answered Feb 11 '11 at 09:30

Scott Langham

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Specialize hashmap template

3 Answers3