Why Do I Still Need to Specify Template Arguments When Using Default Template Arguments in C ?

Understanding the Use of Default Template Arguments in C

Question:

Despite being able to define a class template with default template arguments, as seen below:

<code class="cpp">template <typename T = int>
class Foo {
};</code>

Why is it necessary to specify template arguments when instantiating the class in the following example?

<code class="cpp">Foo<int> me;</code>

Answer:

Contrary to what the question suggests, it is not necessary to explicitly specify template arguments when defining a class template with default template arguments. As of C 17, the following code is valid:

<code class="cpp">Foo me;</code>

Prior to C 17, however, the following syntax was required:

<code class="cpp">Foo<> me;</code>

This means that the template arguments must be present even if they are empty. This behavior is analogous to calling a function with a single default argument, where both foo() and foo without arguments are valid expressions, but only the former will call the function.

By introducing default template arguments, C 11 made it easier to work with class templates by providing a way to set default values for template parameters. Additionally, as demonstrated by the change in behavior between C 11 and C 17, this syntax has evolved over time to improve usability.

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