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Given a template class like the following:
template<typename Type, typename IDType=typename Type::IDType>
class Mappings
{
public:
...
Type valueFor(const IDType& id) { // return value }
...
};
How can someone forward declare this class in a header file?
724
You can declare default arguments for a template only for the first declaration of the template. If you want allow users to forward declare a class template, you should provide a forwarding header. If you want to forward declare someone else's class template using defaults, you are out of luck!
In Objective-C, classes and protocols can be forward-declared if you only need to use them as part of an object pointer type, e.g. MyClass * or id<MyProtocol>.
A class template must be declared before any instantiation of a corresponding template class. A class template definition can only appear once in any single translation unit. A class template must be defined before any use of a template class that requires the size of the class or refers to members of the class.
To write a forward declaration for a function, we use a declaration statement called a function prototype. The function prototype consists of the function header (the function's return type, name, and parameter types), terminated with a semicolon. The function body is not included in the prototype.
This is how you would do it:
template<typename Type, typename IDType=typename Type::IDType>
class Mappings;
template<typename Type, typename IDType>
class Mappings
{
public:
...
Type valueFor(const IDType& id) { // return value }
...
};
Note that the default is in the forward declaration and not in the actual definition.
137
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