Template class with template container
Solution 1
You should use template template parameters:
template<typename T, template <typename, typename> class Container>
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
class MyMultibyteString
{
Container<T, std::allocator<T>> buffer;
// ...
};
This would allow you to write:
MyMultibyteString<int, std::vector> mbs;
Here is a compiling live example. An alternative way of writing the above could be:
template<typename T,
template <typename, typename = std::allocator<T>> class Container>
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
class MyMultibyteString
{
Container<T> buffer; // <== No more need to specify the second argument here
// ...
};
And here is the corresponding live example.
The only thing you have to pay attention to is that the number and type of arguments in the template template parameter declaration must match exactly the number and type of arguments in the definition of the corresponding class template you want to pass as a template argument, regardless of the fact that some of those parameters may have default values.
For instance, the class template std::vector
accepts two template parameters (the element type and the allocator type), although the second one has the default value std::allocator<T>
. Because of this, you could not write:
template<typename T, template <typename> class Container>
// ^^^^^^^^
// Notice: just one template parameter declared!
class MyMultibyteString
{
Container<T> buffer;
// ...
};
// ...
MyMultibyteString<int, std::vector> mbs; // ERROR!
// ^^^^^^^^^^^
// The std::vector class template accepts *two*
// template parameters (even though the second
// one has a default argument)
This means that you won't be able to write one single class template that can accept both std::set
and std::vector
as a template template parameter, because unlike std::vector
, the std::set
class template accepts three template parameters.
Solution 2
Another approach to solve this is by using variadic templates and with that you can use any container as suggested in comments above and here is the implemenation :
template<template <typename... Args> class Container,typename... Types>
class Test
{
public:
Container<Types...> test;
};
int main()
{
Test<std::vector,int> t;
Test<std::set,std::string> p;
return 0;
}
DuXeN0N
Updated on July 09, 2022Comments
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DuXeN0N almost 2 years
How can I declare template class (adaptor) with different containers as template arguments? For example, I need to declare class:
template<typename T, typename Container> class MyMultibyteString { Container buffer; ... };
And I want it to my based on vector. How to make it hard-defined? (to prevent someone from writing such declaration
MyMultibyteString<int, vector<char>>
).Moreover, how to implement such construction:
MyMultibyteString<int, std::vector> mbs;
without passing template argument to container.
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Scott Jones almost 11 yearsWhat a great, thorough answer.
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Andy Prowl almost 11 years@ScottJones: Glad you found it useful :)
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Antonio over 10 years@ScottJones Regarding your statement:
This means that you won't be able to write one single class template that can accept both std::set and std::vector
: Would variadic templates solve the problem? stackoverflow.com/a/20499809/2436175 -
Andy Prowl over 10 years@Antonio: Yep, that's a possibility. Not sure why I haven't thought of that :)
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iPherian over 7 years"This means that you won't be able to write one single class template that can accept both
std::set
andstd::vector
...becausestd::set
accepts three template parameters" - What about variadic templates?template< class... Args >
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newandlost over 5 yearsand how would a function look like that return a
Container<Types...>
?