std::optional (C++17)

In a Nutshell

A container for representing "a value that may not exist" — safer and more intuitive than returning a bool plus a pointer or an output parameter.

Header

#include <optional>

Core API Cheat Sheet

Operation	Signature	Description
Construct	optional()	Default construction; contains no value
Assign empty	optional& operator=(nullopt_t)	Sets the state to no value
Check for value	explicit operator bool() const	Returns true when a value is present
Check for value	bool has_value() const	Same as above
Access value	T& operator*()	Dereferences to obtain the value (undefined behavior when there is no value)
Safe access	T& value()	Returns the value; throws bad_optional_access if there is none
Value or default	T value_or(const T& default_value) const	Returns the value if present, otherwise the default value
In-place construct	T& emplace(Args&&... args)	Constructs the value in place
Reset	void reset() noexcept	Destroys the contained value

Minimal Example

#include <iostream>
#include <optional>
#include <string>

std::optional<std::string> find(bool b) {
    return b ? std::optional<std::string>{"found"} : std::nullopt;
}

int main() {
    auto res = find(false);
    std::cout << res.value_or("not found") << '\n';

    if (auto val = find(true))
        std::cout << *val << '\n';
}

Embedded Applicability: High

• Zero-overhead abstraction: the no-value state takes up only a bool-sized amount of storage, and no heap allocation is involved
• Can replace raw pointers as the return type of functions that might fail, avoiding the risk of dereferencing a null pointer
• Fully supported since C++17; member functions become fully constexpr from C++23 onward, further widening the range of applicable scenarios

Compiler Support

GCC	Clang	MSVC
TBD	TBD	TBD

See Also

• cppreference: std::optional

---

Some content is adapted from cppreference.com, used under the CC-BY-SA 4.0 license