Difference between revisions of "cpp/language/static cast"

#include <iostream>
#include <vector>
 
struct B
{
    int m = 42;
    const char* hello() const
    {
        return "Hello world, this is B!\n";
    }
};
 
struct D : B
{
    const char* hello() const
    {
        return "Hello world, this is D!\n";
    }
};
 
enum class E { ONE = 1, TWO, THREE };
enum EU { ONE = 1, TWO, THREE };
 
int main()
{
    // 1. static downcast
    D d;
    B& br = d; // upcast via implicit conversion
    std::cout << "1) " << br.hello();
    D& another_d = static_cast<D&>(br); // downcast
    std::cout << "1) " << another_d.hello();
 
    // 2. lvalue to xvalue
    std::vector<int> v0{1,2,3};
    std::vector<int> v2 = static_cast<std::vector<int>&&>(v0);
    std::cout << "2) after move, v0.size() = " << v0.size() << '\n';
 
    // 3. initializing conversion
    int n = static_cast<int>(3.14);
    std::cout << "3) n = " << n << '\n';
    std::vector<int> v = static_cast<std::vector<int>>(10);
    std::cout << "3) v.size() = " << v.size() << '\n';
 
    // 4. discarded-value expression
    static_cast<void>(v2.size());
 
    // 5. inverse of implicit conversion
    void* nv = &n;
    int* ni = static_cast<int*>(nv);
    std::cout << "5) *ni = " << *ni << '\n';
 
    // 6. array-to-pointer followed by upcast
    D a[10];
    [[maybe_unused]]
    B* dp = static_cast<B*>(a);
 
    // 7. scoped enum to int
    E e = E::TWO;
    int two = static_cast<int>(e);
    std::cout << "7) " << two << '\n';
 
    // 8. int to enum, enum to another enum
    E e2 = static_cast<E>(two);
    [[maybe_unused]]
    EU eu = static_cast<EU>(e2);
 
    // 9. pointer to member upcast
    int D::*pm = &D::m;
    std::cout << "9) " << br.*static_cast<int B::*>(pm) << '\n';
 
    // 10. void* to any type
    void* voidp = &e;
    [[maybe_unused]]
    std::vector<int>* p = static_cast<std::vector<int>*>(voidp);
}