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C++17 in Detail: A Deep Dive

Ambiguity and Complex Types

What if we want to create objects of complex types? What if there's ambiguity when you initialize std::variant? Read below to find out the answers.

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Let’s Discuss Ambiguity

What if you have an initialization like:

C++
std::variant<int, float> intFloat { 10.5 }; // conversion from double?

The value 10.5 could be converted to int or float, and the compiler doesn’t know which conversion should be applied. It might report a few pages of compiler errors, however.

But you can easily handle such errors by specifying which type you’d like to create:

C++
std::variant<int, float> intFloat { std::in_place_index<0>, 10.5f };
// or
std::variant<int, float> intFloat { std::in_place_type<int>, 10.5f };

Let’s Discuss Complex Types

Similarly to std::optional, if you want to efficiently create objects that require several constructor arguments - then use std::in_place_index or std::in_place_type:

For example:

C++
std::variant<std::vector<int>, std::string> vecStr 
{
  std::in_place_index<0>, { 0, 1, 2, 3 } // initializer list passed into vector
};
...