GSL/include/gsl.h

///////////////////////////////////////////////////////////////////////////////
//
// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
//
// This code is licensed under the MIT License (MIT).
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
///////////////////////////////////////////////////////////////////////////////

#pragma once

#ifndef GSL_GSL_H
#define GSL_GSL_H

#include "gsl_assert.h"  // Ensures/Expects
#include "gsl_util.h"    // finally()/narrow()/narrow_cast()...
#include "multi_span.h"  // multi_span, strided_span...
#include "span.h"        // span
#include "string_span.h" // zstring, string_span, zstring_builder...
#include <memory>

#ifdef _MSC_VER

// No MSVC does constexpr fully yet
#pragma push_macro("constexpr")
#define constexpr

// MSVC 2013 workarounds
#if _MSC_VER <= 1800
// noexcept is not understood
#pragma push_macro("noexcept")
#define noexcept

// turn off some misguided warnings
#pragma warning(push)
#pragma warning(disable : 4351) // warns about newly introduced aggregate initializer behavior

#endif // _MSC_VER <= 1800

#endif // _MSC_VER

namespace gsl
{

//
// GSL.owner: ownership pointers
//
using std::unique_ptr;
using std::shared_ptr;

template <class T>
using owner = T;

//
// not_null
//
// Restricts a pointer or smart pointer to only hold non-null values.
//
// Has zero size overhead over T.
//
// If T is a pointer (i.e. T == U*) then
// - allow construction from U* or U&
// - disallow construction from nullptr_t
// - disallow default construction
// - ensure construction from U* fails with nullptr
// - allow implicit conversion to U*
//
template <class T>
class not_null
{
    static_assert(std::is_assignable<T&, std::nullptr_t>::value, "T cannot be assigned nullptr.");

public:
    not_null(T t) : ptr_(t) { ensure_invariant(); }
    not_null& operator=(const T& t)
    {
        ptr_ = t;
        ensure_invariant();
        return *this;
    }

    not_null(const not_null& other) = default;
    not_null& operator=(const not_null& other) = default;

    template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
    not_null(const not_null<U>& other)
    {
        *this = other;
    }

    template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
    not_null& operator=(const not_null<U>& other)
    {
        ptr_ = other.get();
        return *this;
    }

    // prevents compilation when someone attempts to assign a nullptr
    not_null(std::nullptr_t) = delete;
    not_null(int) = delete;
    not_null<T>& operator=(std::nullptr_t) = delete;
    not_null<T>& operator=(int) = delete;

    T get() const
    {
#ifdef _MSC_VER
        __assume(ptr_ != nullptr);
#endif
        return ptr_;
    } // the assume() should help the optimizer

    operator T() const { return get(); }
    T operator->() const { return get(); }

    bool operator==(const T& rhs) const { return ptr_ == rhs; }
    bool operator!=(const T& rhs) const { return !(*this == rhs); }
private:
    T ptr_;

    // we assume that the compiler can hoist/prove away most of the checks inlined from this
    // function
    // if not, we could make them optional via conditional compilation
    void ensure_invariant() const { Expects(ptr_ != nullptr); }

    // unwanted operators...pointers only point to single objects!
    // TODO ensure all arithmetic ops on this type are unavailable
    not_null<T>& operator++() = delete;
    not_null<T>& operator--() = delete;
    not_null<T> operator++(int) = delete;
    not_null<T> operator--(int) = delete;
    not_null<T>& operator+(size_t) = delete;
    not_null<T>& operator+=(size_t) = delete;
    not_null<T>& operator-(size_t) = delete;
    not_null<T>& operator-=(size_t) = delete;
};

} // namespace gsl

namespace std
{
template <class T>
struct hash<gsl::not_null<T>>
{
    size_t operator()(const gsl::not_null<T>& value) const { return hash<T>{}(value); }
};

} // namespace std

#ifdef _MSC_VER

#undef constexpr
#pragma pop_macro("constexpr")

#if _MSC_VER <= 1800

#undef noexcept
#pragma pop_macro("noexcept")

#pragma warning(pop)

#endif // _MSC_VER <= 1800

#endif // _MSC_VER

#endif // GSL_GSL_H