Why Can't C Compilers Deduce Template Type Parameters from Default Function Arguments?

Why Can't Compilers Deduct Template Type Parameters from Default Function Arguments?

Despite the seemingly logical assumption, C compilers cannot automatically infer template type parameters from default function arguments. This holds true both in C 03 and C 11, for distinct reasons.

In C 03, the compiler's inability stems from explicit language specifications (§14.8.2/17): "A template type-parameter cannot be deduced from the type of a function default argument."

In C 11, while it is possible to specify a default template argument, it must be provided explicitly. The default function argument itself remains unusable for template argument deduction:

<code class="cpp">void bar(int a, T b = 0.0f) { } // C++11</code>

The C 11 standard (14.8.2.5/5) defines non-deduced contexts, which include:

• "A template parameter used in the parameter type of a function parameter that has a default argument that is being used in the call for which argument deduction is being done."

As a result, the explicit provision of default template arguments is often necessary to avoid compilation errors.

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