How to avoid decay with template parameter deduction

问题

Simplified:

// CHAR_TYPE == char, wchar_t, ...
template <typename CHAR_TYPE, unsigned CHAR_COUNT>
void Foo(CHAR_TYPE const (&value)[CHAR_COUNT]) noexcept
{
    TRACE("const ref array");
    // perform a bit of logic and forward...
}

template <typename CHAR_TYPE>
void Foo(CHAR_TYPE const* value) noexcept
{
    TRACE("const ptr");
    // perform a bit of logic and forward...
}

// [ several other overloads ]

Callsite:

char const* ptr = ...
wchar_t const* wptr = ...

Foo(ptr);     // <-- good: "const ptr"
Foo(wptr);    // <-- good: "const ptr"

constexpr char const buffer[] { "blah blah blah" };
constexpr wchar_t const wbuffer[] { L"blah blah blah" };

Foo(buffer);  // <-- ambiguous
Foo(wbuffer); // <-- ambiguous

Of course, I could remove the const ref array overload. However I would like to handle these types differently. I have tried to conditionally enable the correct overload, but I have not been able to determine the necessary condition.

template <typename CHAR_TYPE, unsigned COUNT>
typename std::enable_if</* std::is_?? */, void>::type
    Foo(CHAR_TYPE const (&value)[COUNT]) noexcept
{
    TRACE("by ref array");
    // perform a bit of logic and forward...
}

template <typename CHAR_TYPE>
typename std::enable_if</* std::is_?? */, void>::type
    Foo(CHAR_TYPE const* value) noexcept
{
    TRACE("ptr");
    // perform a bit of logic and forward...
}

What is the best way to disambiguate these overloads?
(I would prefer not to use an array wrapper)

回答1:

Taking the argument by (const) reference blocks array-to-pointer decay during template argument deduction. See [temp.deduct.call]/2. So:

template <typename CHAR_TYPE>
void Foo(CHAR_TYPE const* const & value) noexcept
{
    TRACE("const ptr");
    // perform a bit of logic and forward...
}

回答2:

One idea that works is to remove the pointer and simply have T instead, with a std::enable_if_t<std::is_pointer<T>::value> guard. Simplified example below:

#include <iostream>
#include <type_traits>

template<class T, size_t N>
void f(T const (&) [N])
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

template<class T, std::enable_if_t<std::is_pointer<T>::value>* = nullptr >
void f(T)
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

int main()
{
    const char* str = "test";
    char str2[]{"test2"};

    f(str);
    f(str2);
}

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来源：https://stackoverflow.com/questions/41290958/how-to-avoid-decay-with-template-parameter-deduction

标签

c++

c++11

templates

sfinae