std::in_place_type and std::in_place_index in C++

In modern C++, std::in_place_type and std::in_place_index serve as disambiguation tags that enable direct, in-place construction of objects inside container-like wrappers such as std::variant, std::optional, and std::any. Introduced in C++17, these utility tags allow developers to pass constructor arguments directly to the contained object instead of creating and copying or moving a temporary instance. This article explores how these tags work, why they are essential for performance and non-movable types, and how they resolve constructor ambiguities in variant types.

The Purpose of In-Place Construction

When initializing a wrapper like std::optional or std::variant, passing a pre-constructed object requires that object to be either copied or moved into the wrapper's internal storage:

std::vector<int> data = {1, 2, 3};
std::optional<std::vector<int>> opt(data); // Incurs a copy

To eliminate this overhead, C++ provides in-place construction. Passing an in-place tag informs the wrapper's constructor that the remaining arguments should be forwarded directly to the constructor of the contained type. This avoids temporaries, improves efficiency, and allows the initialization of non-copyable and non-movable types (such as std::mutex).

Using std::in_place_type

std::in_place_type is a tag template that specifies the exact type to construct inside the target wrapper. It is commonly used with std::variant to specify which alternative to instantiate, particularly when the type requires multiple constructor arguments.

#include <variant>
#include <string>
#include <vector>

int main() {
    // Directly constructs std::string with 5 occurrences of 'a'
    std::variant<int, std::string, std::vector<int>> v(
        std::in_place_type<std::string>, 5, 'a'
    );
}

Without std::in_place_type, creating this variant would require constructing a temporary std::string(5, 'a') and moving it into the variant.

Although std::optional typically uses the simpler std::in_place tag because it holds only one type, std::in_place_type<T> is also accepted.

Using std::in_place_index

std::in_place_index is a tag template that identifies the alternative in a std::variant by its zero-based position rather than its type.

This tag is essential in scenarios where std::in_place_type cannot be used:

1. Duplicate Types: A std::variant can contain multiple alternatives of the same type. In this case, specifying a type is ambiguous.
2. Ambiguous Conversions: When several alternatives can accept the provided constructor arguments, specifying the index removes any compiler ambiguity.

#include <variant>
#include <string>

int main() {
    // Variant containing duplicate types
    using DuplicateVariant = std::variant<std::string, std::string>;

    // Constructs the second std::string (index 1) directly in place
    DuplicateVariant v(std::in_place_index<1>, "Second Alternative");
}

Summary of Benefits

• Performance: Arguments are forwarded directly to the target constructor, eliminating unnecessary copy and move operations.
• Support for Immovable Types: Enables containers like std::optional and std::variant to hold types that disable both copy and move constructors.
• Disambiguation: std::in_place_type clearly states the intended type for multi-argument constructors, while std::in_place_index solves ambiguity for variants with repeated or convertible types.