Builder pattern with struct - c++

Question

I'm trying to implement a builder patter in C++. That what i got by now:
struct Person {
std::string name;
uint32_t age;
std::vector<std::string> pet_names;
};
class PersonBuilder {
public:
PersonBuilder& SetName(std::string name) {
person_.name = std::move(name);
return *this;
}
PersonBuilder& SetAge(uint32_t age) {
person_.age = age;
return *this;
}
PersonBuilder& SetPetNames(std::vector<std::string> pet_names) {
person_.pet_names = std::move(pet_names);
return *this;
}
Person Build() {
return Person(std::move(person_));
}
private:
Person person_;
};
int main() {
auto person = PersonBuilder()
.SetName("John")
.SetAge(23)
.SetPetNames({"rax", "rbx", "rcx"})
.Build();
return EXIT_SUCCESS;
}
I'm a little worried about unitilized uint32, and std::move(person_) inside a Build method. Am i doing it right or there is a better way, with a struct constructor for example?

Answer 1

I'm a little worried about unitilized uint32
You initialise it before you use it, so there is no problem in the example. It is however easy for a user of the builder to forget calling one of the setters leaving it with indeterminate value resulting in undefined behaviour. You give it a default member initialiser to avoid that.
and std::move(person_) inside a Build method.
It's unnecessary to repeat the name of the class there. This would also work:
return std::move(person_);
It's also important for the caller to understand that Build may only be called once per initialisation. If the builder is intended to only be used as a temporary like in the example, you could add rvalue reference qualifier to the Build function to prevent some of the accidental misuse.
or there is a better way,
I don't see the benefit of the builder. Why not just use aggregate initialisation?:
Person person {
.name = "John",
.age = 23,
.pet_names = {"rax", "rbx", "rcx"},
};

Answer 2

You did it right.
You can also turn the set function into a template function, which can avoid the modification of PersonBuilder when adding new member variables for Person.
struct Person {
std::string name;
uint32_t age;
std::vector<std::string> pet_names;
};
class PersonBuilder {
public:
template<auto mem_ptr>
PersonBuilder& Set(std::remove_reference_t<std::invoke_result_t<decltype(mem_ptr), Person&>> value = {}) {
std::invoke(mem_ptr, person_) = std::move(value);
return *this;
}
Person Build() {
auto result = std::move(person_);
person_ = Person();
return result;
}
private:
Person person_;
};
int main() {
auto person = PersonBuilder()
.Set<&Person::name>("John")
.Set<&Person::age>(23)
.Set<&Person::pet_names>({"rax", "rbx", "rcx"})
.Build();
}
Demo.