static_cast conversion

Converts between types using a combination of implicit and user-defined conversions.

Syntax

Returns a value of type target-type.

Explanation

Only the following conversions can be done with static_cast, except when such conversions would cast away constness (or volatility).

struct B {};
struct D : B { B b; };
 
D d;
B& br1 = d;
B& br2 = d.b;
 
static_cast<D&>(br1); // OK: lvalue denoting the original d object
static_cast<D&>(br2); // UB: the b subobject is not a base class subobject

• If the underlying type is not fixed, the behavior is undefined if the value of expression is out of range (the range is all values possible for the smallest bit-field large enough to hold all enumerators of the target enumeration).
• If the underlying type is fixed, the result is the same as converting the original value first to the underlying type of the enumeration and then to the enumeration type.

• The result is the same as converting the original value first to the underlying type of the enumeration, and then to the enumeration type.

• If the original pointer value represents an address of a byte in memory that does not satisfy the alignment requirement of T, then the resulting pointer value is unspecified.
• Otherwise, if the original pointer value points to an object a, and there is an object b of type similar to T that is pointer-interconvertible with a, the result is a pointer to b.
• Otherwise the pointer value is unchanged.

As with all cast expressions, the result is:

• an lvalue if target-type is an lvalue reference type or an rvalue reference to function type(since C++11);

• a prvalue otherwise.

Pointer-interconvertible objects

Two objects a and b are pointer-interconvertible if:

• they are the same object, or
• one is a union object and the other is a non-static data member of that object, or
• one is a standard-layout class object and the other is the first non-static data member of that object or any base class subobject of that object, or
• there exists an object c such that a and c are pointer-interconvertible, and c and b are pointer-interconvertible.

union U { int a; double b; } u;
void* x = &u;                        // x's value is "pointer to u"
double* y = static_cast<double*>(x); // y's value is "pointer to u.b"
char* z = static_cast<char*>(x);     // z's value is "pointer to u"

Notes

static_cast may also be used to disambiguate function overloads by performing a function-to-pointer conversion to specific type, as in

std::for_each(files.begin(), files.end(),
              static_cast<std::ostream&(*)(std::ostream&)>(std::flush));

Keywords

static_cast

Example

#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);
}

1) Hello world, this is B!
1) Hello world, this is D!
2) after move, v0.size() = 0
3) n = 3
3) v.size() = 10
5) *ni = 3
7) 2
9) 42

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also

• const_cast
• dynamic_cast
• reinterpret_cast
• explicit cast
• implicit conversions