In Place Construction
std::variant has two in_place helpers which this lesson elaborates on. Let's read more about them below!
We'll cover the following
std::variant and in_place helpers
std::variant
has two in_place
helpers that you can use:
• std::in_place_type
- used to specify which type you want to change/set in the variant
• std::in_place_index
- used to specify which index you want to change/set. Types are
enumerated from 0.
- In a variant
std::variant<int, float, std::string>
-int
has the index 0,float
has index 1 and thestring
has index of 2. The index is the same value as returned fromvariant::index
method.
Fortunately, you don’t always have to use the helpers to create a variant. It’s smart enough to recognize if it can be constructed from the passed single parameter:
// this constructs the second float
std::variant<int, float, std::string> intFloatString { 10.5f };
For variant we need the helpers for at least two cases:
• ambiguity - to distinguish which type should be created where several could match
• efficient complex type creation (similar to optional)
Get hands-on with 1200+ tech skills courses.