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Ben's Blog C++ Detection Idiom Through the Years

This is a record, mostly for my own benefit, of different ways to write the detection idiom in C++. If you're looking for a tutorial-style guide to the detection idiom, I highly recommend Sy Brand's blog post about the detection idiom as a stand-in(替代) for concepts.

What can detection idiom do?

The detection idiom is what allows compile-time type introspection in C++. Using the detection idiom, we can check if a type has methods, type aliases, or members that fit a particular interface. This allows library writers to create metaprogramming facilities that are much more expressive. The user can plug(插入) any type she likes into a metafunction as long as it fits the required interface.

C++ 20 concepts

C++ 20 will have a first-class facility for compile-type type introspection called concepts. Concepts are more concise and explicit than current iterations of the detection idiom and will result in better compiler errors. For now, though, we emulate concepts using the detection idiom. All current iterations of the detection idiom depend on SFINAE, which is the property that certain errors that occur during overload resolution are not fatal errors, but just result in substitution failure, with the compiler going on to try the next candidate. We can use this property to check whether a type has a particular interface, returning true if it does and false if it does not.

NOTE:

1、上面这段话中关于SFINAE的描述是有些狭隘的，"which is the property that certain errors that occur during overload resolution are not fatal errors"，SFINAE不仅仅发生于overload resolution，相反，它是发生于所有的template instantiation

Detection idiom implementation

Modern Detection Idiom: Use Fundamentals TS v2

C++ Fundamentals TS v2 provides a super easy way to use the detection idiom in C++ through is_detected. All the user needs to do is write a template struct that checks for the attributes the user wants, then hand that off to is_detected. Here's an example, where we want to ensure that a type has a method get()

template<typename T>
using get_t = decltype(std::declval<T>().get());

template<typename >
using supports_get = std::experimental::is_detected<get_t, T>;

main()
{

    std::cout << supports_get<foo>::value << std::endl;

    ...
}

You can use your new supports_get template metafunction to create partial specializations, SFINAE out, etc. You can use conjunction to check for multiple traits and is_same to check for the return value. Both of these are in type_traits. The downside of this is that either your compiler needs to include <experimental/type_traits>, or you need to include the the implementation of is_detected from cppreference.

C++11 Detection Idiom: Use Expression SFINAE

In an older style of the detection idiom, you create a single class and use expression SFINAE on the return type of methods in that class to return true if the method is detected and false if the method does not exist. In the example below, we're calling test_get with an int parameter. This causes the first test_get method, which takes an int as an argument, to be preferred over the second test_get method, which takes anything (...) as an argument. The first method will only be enabled, however, if the expression inside the decltype can resolve without errors. The return type for both functions is bool, since decltype will return the type of the last expression (technically, this is due to the comma operator).

NOTE:

1、理解上面这段话的一个前提是理解C++编译流程，参见Compile-flow章节

2、关于comma operator，参见 Built-in-comma-operator 章节

3、关于ellipsis operator，参见 Ellipsis-catch-all-operator 章节

template<typename T>
struct has_get
{

    template<typename U>
    static constexpr
    decltype(std::declval<U>().get(), bool()) test_get(int)
    {
        return true;
    }

    template<typename U>
    static constexpr bool test_get(...)
    {
        return false;
    }

    static constexpr bool value = test_get<T>(int());
};

Pre-C++11 Detection Idiom:

If we restrict ourselves to pre-C++11, we lose decltype, which is the main driver of the detection idiom patterns above. We can, however, fairly easily emulate this by abusing sizeof.

template<typename T>
T declval();

template<typename T>
struct has_get
{
    typedef char yes[1];
    typedef char no[2];

    template<typename U>
    static yes& test_get(int (*)[sizeof(declval<U>().get(), 1)]);

    template<typename U>
    static no& test_get(...);

    static const bool value = sizeof(test_get<T>(NULL)) == sizeof(yes);
};

NOTE:

1、上述implementation是非常值得借鉴学习的

Note that here we're using the size of the return value to check how the overloaded test_get function is resolved. Also, we have to define our own declval, since that does not exist pre-C++11. In the first test_get, we're passing a pointer to a fixed size array int (*) [x], where x, the size of the array, is determined by our sizeof expression. Similar to the decltype expressions above, this will SFINAE out if our type does not have the method get(), and will return 1 otherwise.