Aliases

You can use an alias declaration to declare a name to use as a synonym for a previously declared type. (This mechanism is also referred to informally as a type alias). You can also use this mechanism to create an alias template, which can be particularly useful for custom allocators.

using identifier = type;

Remarks

• identifier
  The name of the alias.

• type
  The type identifier you are creating an alias for.

An alias does not introduce a new type and cannot change the meaning of an existing type name.

The simplest form of an alias is equivalent to the typedef mechanism from C++03:

// C++11
using counter = long;

// C++03 equivalent:
// typedef long counter;

Both of these enable the creation of variables of type "counter". Something more useful would be a type alias like this one for std::ios_base::fmtflags:

// C++11
using fmtfl = std::ios_base::fmtflags;
// C++03 equivalent:
// typedef std::ios_base::fmtflags fmtfl;

fmtfl fl_orig = std::cout.flags();
fmtfl fl_hex = (fl_orig & ~std::cout.basefield) | std::cout.showbase | std::cout.hex;
// ...
std::cout.flags(fl_hex);

Aliases also work with function pointers, but much more readable than typedef's equivalent:

// C++11
using func = void(*)(int);

// C++03 equivalent:
// typedef void (*func)(int);

// func can be assigned to a function pointer value
void actual_function(int arg) { /* some code */ }
func fptr = &actual_function;

A limitation of the typedef mechanism is that it doesn't work with templates. However, the type alias syntax in C++11 enables the creation of alias templates:

template<typename T> using ptr = T*; 

// the name 'ptr<T>' is now an alias for pointer to T
ptr<int> ptr_int;

Example

The following example demonstrates how to use an alias template with a custom allocator—in this case, an integer vector type. You can substitute any type for int to create a convenient alias to hide the complex parameter lists in your main functional code. By using the custom allocator throughout your code you can improve readability and reduce the risk of introducing bugs caused by typos.

#include <stdlib.h>
#include <new>

template <typename T> struct MyAlloc {
    typedef T value_type;

    MyAlloc() { }
    template <typename U> MyAlloc(const MyAlloc<U>&) { }

    bool operator==(const MyAlloc&) const { return true; }
    bool operator!=(const MyAlloc&) const { return false; }

    T * allocate(const size_t n) const {
        if (n == 0) {
            return nullptr;
        }

        if (n > static_cast<size_t>(-1) / sizeof(T)) {
            throw std::bad_array_new_length();
        }

        void * const pv = malloc(n * sizeof(T));

        if (!pv) {
            throw std::bad_alloc();
        }

        return static_cast<T *>(pv);
    }

    void deallocate(T * const p, size_t) const {
        free(p);
    }
};

#include <vector>
using MyIntVector = std::vector<int, MyAlloc<int>>;

#include <iostream>

int main () 
{
    MyIntVector foov = { 1701, 1764, 1664 };

    for (auto a: foov) std::cout << a << " ";
    std::cout << "\n";

    return 0;
}

Output

1701 1764 1664

See Also

Reference

using Keyword