A dependent name is essentially a name that depends on a template parameter. A dependent name can be a type, a non-type, or a template parameter. To express that a dependent name stands for a type or a template, you have to use the keywords
Before I write about dependent names, I should write about template parameters.
Template parameters can be types, non-types, and templates.
Types are the most often used template parameters. Here are a few examples:
Non-types can be a
- lvalue reference
- integral types
- floating-point types (C++20)
- literal types (C++20)
Non-type template parameters are often just called NTTP.
Integrals are the most used non-types. std::array is the typical example because you have to specify at compile time the size of a std::array:
With C++20, you can also use two new non-types: floating-point types and literal types.
Literal Types must have the following two properties, essentially:
- All base classes and non-static data members are public and non-mutable
- The types of all base classes and non-static data members are structural types or arrays of these
A literal type must have a
The following program uses floating-point types and literal types as template parameters.
ClassType has a
constexpr constructor (1) and can be used as a template argument (2). The function template
getDouble (3) accepts only doubles. I want to emphasize it explicitly that each call of the function template
getDouble (4) with a new argument triggers the instantiation of a new function template specialization of
getDouble. This means that two instantiations for the doubles 5.5 and 6.5 are created.
Templates can be template parameters; in this case, they are called template template parameters.
Matrix is a simple class template that can be initialized by a std::initializer_list (line 2). A Matrix can be used with a std::vector (line 4 and line 5) or a std::list (line 6) to hold its values. So far, nothing special.
Line 1 declares a class template that has two template parameters. The first parameter is the type of the elements, and the second parameter stands for the container. Look at the second parameter: template <typename, typename> class Cont>. This means the second template argument should be a template requiring two template parameters. The first template parameter is the type of elements the container stores, and the second is the defaulted allocator a container of the standard template library has. The allocator has a default value, such as in the case of a std::vector. The allocator depends on the type of elements.
Line 3 shows the usage of the allocator in this internally used container. The matrix can be instantiated with all containers of the kind: container< type of the elements, allocator of the elements>. This is true for the sequence containers such as std::vector, std::deque, or std::list. std::array and std::forward_list would fail because std::array needs an additional non-type for specifying its size at compile-time, and std::forward_list does not support the size method.
Now, I can write about dependent names.
What is a dependent name? A dependent name is essentially a name that depends on a template parameter. Here are a few examples based on cppreference.com:
Now, the fun starts. A dependent name can be a type, a non-type, or a template parameter. The name lookup is the big difference between non-dependent and dependent names.
- Non-dependent names are looked up at the point of the template definition.
- Dependent names are looked up at the point of the template instantiation.
When you use a dependent name in a template declaration, the compiler has no idea whether this name refers to a type, a non-type, or a template parameter. In this case, the compiler assumes that the dependent name refers to a non-type, which may be wrong. This is when you have to give the compiler a hint.
Before I show you two examples, I must write about an exception to this rule. You can skip my next few words if you want to get a general idea and jump directly to the next section. The exception is: if the name refers in the template definition to the current instantiation, the compiler can deduce the name at the point of the template definition. Here are a few examples:
Finally, I come to the critical idea of my post. If a dependent name could be a type, a non-type, or a template, you have to give the compiler a hint.
Use typename if the Dependent Name is a Type
After such a long introduction, the following program snippet makes it pretty clear.
Without the typename keyword in line 2, the name std::vector<T>::const_iterator in line 2 would be interpreted as a non-type and, consequently, the * stands for multiplication and not for a pointer declaration. Exactly this is happening in line (1).
Similarly, if your dependent name should be a template, you must give the compiler a hint.
Use .template if the Dependent Name is a Template
Honestly, this syntax looks a bit weird.
Same story as before. Compare lines 1 and 2. When the compiler reads the name s.func (line 1), it interprets it as non-type. This means the < sign stands for the comparison operator, but not opening the square bracket of the template argument of the generic method func. In this case, you must specify that s.func is a template, such as in line 2: s.template func.
Here is the essence of this post in one sentence: When you have a dependent name, use
typename to specify that the dependent name is a type or use
.template to specify that the dependent name is a template.
In my next post, I will present automatic return types. They are often a lifesaver when it comes to function templates.
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