Converts between types by reinterpreting the underlying bit pattern.

[edit]Syntax

Returns a value of typetarget-type.

[edit]Explanation

Unlikestatic_cast, but likeconst_cast, thereinterpret_cast expression does not compile to any CPU instructions (except when converting between integers and pointers, or between pointers on obscure architectures where pointer representation depends on its type). It is primarily a compile-time directive which instructs the compiler to treatexpression as if it had the typetarget-type.

Only the following conversions can be done withreinterpret_cast, except when such conversions wouldcast away constness (or volatility).

As with all cast expressions, the result is:

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

• a prvalue otherwise.

[edit]Type aliasing

[edit]Type accessibility

If a typeT_ref issimilar to any of the following types, an object ofdynamic typeT_obj istype-accessible through alvalue(until C++11)glvalue(since C++11) of typeT_ref:

• char,unsignedchar orstd::byte(since C++17): this permits examination of theobject representation of any object as an array of bytes.
• T_obj
• the signed or unsigned type corresponding toT_obj

If a program attempts to read or modify the stored value of an object through alvalue(until C++11)glvalue(since C++11) through which it is not type-accessible, the behavior is undefined.

This rule enables type-based alias analysis, in which a compiler assumes that the value read through a glvalue of one type is not modified by a write to a glvalue of a different type (subject to the exceptions noted above).

Note that many C++ compilers relax this rule, as a non-standard language extension, to allow wrong-type access through the inactive member of aunion (such access is not undefined in C).

[edit]Call compatibility

If any of the following conditions is satisfied, a typeT_call iscall-compatible with a function typeT_func:

• T_call is the same type asT_func.

If a function is called through an expression whosefunction type is not call-compatible with the type of the called function’s definition, the behavior is undefined.

[edit]Notes

Assuming that alignment requirements are met, areinterpret_cast does not change thevalue of a pointer outside of a few limited cases dealing withpointer-interconvertible objects:

struct S1{int a;} s1;struct S2{int a;private:int b;} s2;// not standard-layoutunion U{int a;double b;} u={0};int arr[2]; int* p1=reinterpret_cast<int*>(&s1);// value of p1 is "pointer to s1.a" because// s1.a and s1 are pointer-interconvertible int* p2=reinterpret_cast<int*>(&s2);// value of p2 is unchanged by reinterpret_cast// and is "pointer to s2". int* p3=reinterpret_cast<int*>(&u);// value of p3 is "pointer to u.a":// u.a and u are pointer-interconvertible double* p4=reinterpret_cast<double*>(p3);// value of p4 is "pointer to u.b": u.a and// u.b are pointer-interconvertible because// both are pointer-interconvertible with u int* p5=reinterpret_cast<int*>(&arr);// value of p5 is unchanged by reinterpret_cast// and is "pointer to arr"

Performing a class member access that designates a non-static data member or a non-static member function on a glvalue that does not actually designate an object of the appropriate type - such as one obtained through areinterpret_cast - results in undefined behavior:

struct S{int x;};struct T{int x;int f();};struct S1: S{};// standard-layoutstruct ST: S, T{};// not standard-layout S s={};auto p=reinterpret_cast<T*>(&s);// value of p is "pointer to s"auto i= p->x;// class member access expression is undefined behavior;// s is not a T objectp->x=1;// undefined behaviorp->f();// undefined behavior S1 s1={};auto p1=reinterpret_cast<S*>(&s1);// value of p1 is "pointer to the S subobject of s1"auto i= p1->x;// OKp1->x=1;// OK ST st={};auto p2=reinterpret_cast<S*>(&st);// value of p2 is "pointer to st"auto i= p2->x;// undefined behaviorp2->x=1;// undefined behavior

Many compilers issue "strict aliasing" warnings in such cases, even though technically such constructs run afoul of something other than the paragraph commonly known as the "strict aliasing rule".

The purpose of strict aliasing and related rules is to enable type-based alias analysis, which would be decimated if a program can validly create a situation where two pointers to unrelated types (e.g., anint* and afloat*) could simultaneously exist and both can be used to load or store the same memory (seethis email on SG12 reflector). Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior.

When it is needed to interpret the bytes of an object as a value of a different type,std::memcpyorstd::bit_cast(since C++20) can be used:

double d=0.1;std::int64_t n;static_assert(sizeof n== sizeof d);// n = *reinterpret_cast<std::int64_t*>(&d); // Undefined behaviorstd::memcpy(&n,&d, sizeof d);// OKn=std::bit_cast<std::int64_t>(d);// also OK

In C, aggregate copy and assignment access the aggregate object as a whole. But in C++ such actions are always performed through a member function call, which accesses the individual subobjects rather than the entire object (or, in the case of unions, copies the object representation, i.e., viaunsignedchar).

[edit]Keywords

reinterpret_cast

[edit]Example

#include <cassert>#include <cstdint>#include <iostream> int f(){return42;} int main(){int i=7; // pointer to integer and backstd::uintptr_t v1=reinterpret_cast<std::uintptr_t>(&i);// static_cast is an errorstd::cout<<"The value of &i is "<<std::showbase<<std::hex<< v1<<'\n';int* p1=reinterpret_cast<int*>(v1);assert(p1==&i); // pointer to function to another and backvoid(*fp1)()=reinterpret_cast<void(*)()>(f);// fp1(); undefined behaviorint(*fp2)()=reinterpret_cast<int(*)()>(fp1);std::cout<<std::dec<< fp2()<<'\n';// safe // type aliasing through pointerchar* p2=reinterpret_cast<char*>(&i);std::cout<<(p2[0]=='\x7'?"This system is little-endian\n":"This system is big-endian\n"); // type aliasing through referencereinterpret_cast<unsignedint&>(i)=42;std::cout<< i<<'\n'; [[maybe_unused]]constint&const_iref= i;// int &iref = reinterpret_cast<int&>(//     const_iref); // compiler error - can't get rid of const// Must use const_cast instead: int &iref = const_cast<int&>(const_iref);}

The value of &i is 0x7fff352c358042This system is little-endian42

[edit]Defect reports

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

[edit]References

[edit]See also

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