CODEWITHSUNDEEP
c++compiler-errorscompile-time
Nikita
Nikita

Reputation: 639

Deduce function overload with uknown type at compile time

I stumbled across this when trying to clean up a temporary variable. It seemed interesting enough to discuss.

ArrayType m_ArrayOfThings;
INT32 m_BitfieldOfThings;

...


// Assume MyType has a ctor overload with argument ArrayType and another with int32.
const MyType my_obj( m_ArrayOfThings.IsEmpty() ? m_BitfieldOfThings : m_ArrayOfThings )

Here is a simplified example of my set-up. Something similar may be found in UE4's FCollisionObjectQueryParams constructors.

Compiling the above will result in error C2446: no conversion from 'INT32' to 'ArrayType', since the type passed is unknown at compile-time.

auto temp = m_ArrayOfThings.IsEmpty() ? m_BitfieldOfThings : m_ArrayOfThings;
const MyType my_obj( temp );

This will also throw: error C3536: 'temp': cannot be used before it is initialized.

It's obvious one should convert either variables to have matching types, so the type is known at compile-time, however, I was wondering if there exist semantics, tricks/hacks, or new features (c++17, etc.) to solve this as well.

Upvotes: 0

Views: 53

Answers (1)

user7860670
user7860670

Reputation: 37495

You can just move constructor invocation to the right:

auto const my_obj{m_ArrayOfThings.IsEmpty() ? MyType{m_BitfieldOfThings} : MyType{m_ArrayOfThings}};

With C++17 this snippet will work even if MyType is non-copyable / non-movable and will invoke only one constructor.

Upvotes: 2

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