C++17 introduces std::optional<T>, a template class that manages an optional contained value. By default, the
container doesn’t contain any value. The contained value can be accessed through member functions like value(), operator*(),
or operator->(). Before accessing the value, it is a good practice to check its presence using has_value() or
operator bool().
value_or(default_value) member function returns the contained value if present or default_value otherwise. This rule
flags patterns that could be simplified by a single call to value_or(default_value) instead of two steps logic:
- check presence, i.e., with
has_value
- use
value() if present, default_value otherwise
Noncompliant code example
void fun(const std::optional<std::string> &arg) {
if (arg.has_value()) { // Noncompliant, the entire if statement can be simplified to a simpler statement using "value_or(default_value)"
std::cout << arg.value();
} else {
std::cout << "magic";
}
// another way to check presence and access value
std::cout << (arg ? *arg : "magic"); // Noncompliant, replace the ternary operator by using "value_or(default_value)"
}
void moveFun(std::optional<std::string> arg) {
if (arg.has_value()) { // Noncompliant, the entire if statement can be simplified to a simpler statement using "value_or(default_value)"
sink(std::move(arg.value()));
} else {
sink("magic");
}
}
Compliant solution
void fun(const std::optional<std::string> &arg) {
std::cout << arg.value_or("magic"); // Compliant, neat and simple
}
void moveFun(std::optional<std::string> arg) {
sink(std::move(arg).value_or("magic"));
}
