Daily bit(e) of C++ | accessing std::optional

Daily bit(e) of C++ ♻️110, Accessing the value stored inside of C++17 std::optional.

The std::optional (C++17) is a container for representing a single optional value.

The class offers several interfaces for accessing and querying the presence of a contained value. The choice depends on your coding style and your desired exception semantics.

#include <optional>

struct Data {
    void method() {}
    int v;
};

std::optional<Data> x{Data{1}};

// Pointer-style noexcept interface
if (x != std::nullopt) {
    x->method();
    auto v = x->v;
    // decltype(v) == int
    // v == 1
}

// Explicit throwing interface
if (x.has_value()) {
    auto y = x.value();
    // decltype(y) == Data
    // y.v == 1
}

std::optional<Data> y;
try {
    y.value(); // throws bad_optional_access
} catch (const std::exception& e) {
    // e.what() == "bad optional access"
}

// Default-value interface, throws only if construction
// of the contained type throws
auto z = x.value_or(Data{2});
// z.v == 1

x = std::nullopt; // or x.reset();
z = x.value_or(Data{2});
// z.v == 2

Open the example in Compiler Explorer.