## Can C 17 Partially Deduce Template Arguments in Class Template Argument Deduction (CTAD)?

Partial Template Argument Deduction in C 17

Class Template Argument Deduction (CTAD) was introduced in C 17, allowing the compiler to automatically deduce template arguments based on the types of function arguments. However, CTAD currently requires all template arguments to be deduced or explicitly specified. Is it possible to partially specify template arguments and let the remaining ones be deduced?

Consider the following example:

<code class="cpp">template<class T, class U, class V>
struct Base {
  constexpr Base(T, U) {}
  constexpr Base(T, U, V) {}
  constexpr Base(V) {}
};

void func() {
  constexpr Base val(1, 4.0, false);
}</code>

Using CTAD, the compiler will correctly deduce that val has the type Base. However, what if we want to partially specify the template arguments, like in the following example?

<code class="cpp"> constexpr Base<T = bool> val1(1, 4.0); // U & V deduced -> Base<int, double, bool>
constexpr  Base<T = bool, T = int> val2(5.0); // V deduced -> Base<bool, int, double></code>

Unfortunately, this code will not compile, as the compiler requires all template arguments to be either deduced or explicitly specified.

Workarounds

Since partial CTAD is not directly supported, there are a few workarounds we can use:

• Create alias templates for specific argument combinations, as shown in the following example:

<code class="cpp">using NewBase2 = Base<double, int>;

void func() {
  constexpr NewBase2 val(1, 2);
}</code>

• Use template metaprogramming techniques to conditionally select the appropriate template based on the partially specified arguments.

Conclusion

Partial CTAD is not directly supported in C 17, but there are workarounds available to achieve similar functionality. The upcoming C 20 standard is expected to include support for CTAD with alias templates, but it does not currently include support for partial CTAD or CTAD with inherited constructors.

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