Why Do I Still Need to Specify Arguments for a Class Template with Default Arguments?

Understanding Default Template Arguments in C : Avoiding Specified Arguments

In C programming, it is possible to declare a class template with default template arguments. However, it can be confusing to understand why specifying arguments is still required in certain scenarios.

Original Question:

Consider the following class template:

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

According to the question, the following code should be allowed:

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

However, instead, the following code is required:

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

Answer:

Prior to C 17, specifying arguments in a class template declaration with default arguments was mandatory. In other words, the following code was required:

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

Even though the template argument is not explicitly provided, the empty angle brackets < > must be present to indicate that the default argument is being used. This syntax is analogous to calling a function with a single default argument, which requires empty parentheses ().

C 17 Update:

In C 17, this behavior changed, and it became possible to omit the angle brackets <> when using the default template argument. Therefore, the following alternative syntax is now allowed:

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

Conclusion:

Understanding the requirement of specifying arguments in class templates with default arguments requires familiarity with the historical evolution of C . In pre-C 17 versions, explicit angle brackets <> were required, while C 17 introduced a more convenient syntax that allows for their omission when using the default argument.

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