有一种方法可以对前向声明（不完整）类型（https://stackoverflow.com/a/72276259/9363996）进行部分模板特化。但是在看到提到的问题之后，我想知道是否可以使用不完整的嵌套（可能是私有）类来为类模板定义部分特化。

目前在 C++ 中无法前向声明没有定义类的嵌套类：

class undefined;
class undefined::foo; // impossibru

通过一些技巧，我制作了一个工作代码（为了研究）：https://godbolt.org/z/9W8nfhx8P

#include <iostream>

template <typename T, typename = T>
struct specialize;

// workaround to get to a nested foo
template <typename T, typename...>
struct mem_foo
{
    // error: 'struct undefined::foo' is private within this context
    using type = typename T::foo;
};

class undefined;
// class undefined::foo; // impossibru

template <typename MemFoo>
struct specialize<typename mem_foo<undefined, MemFoo>::type, MemFoo>
{
    void operator()(const MemFoo &f) const 
    {
        // this will compile however
        std::cout << f.name << std::endl;
    }
};

#include <string>

class undefined
{
private: // will not compile without friend
    struct foo{
        std::string name = "John Cena";
    };

    friend struct mem_foo<undefined, foo>; // ugly 
    // friend struct specialize<foo>; // this is irrelevant, but would be nicer than mem_foo

public:
    static foo get() { return {}; }
};

int main()
{
    specialize</*undefined::foo*/decltype(undefined::get())>{}(undefined::get());

    return 0;
}

但是对于 private 类型，使用了丑陋的朋友。 friend struct specialize<undefined::foo>; 在语义上会更具吸引力。

还有其他或更优雅的解决方案吗？

#include <iostream>

template <typename T, typename = T>
struct specialize;

template <typename T, typename ...>
struct defer_instantiation
{
    using type = T;
};

template <typename T, typename ... R>
using defer_instantiation_t = typename defer_instantiation<T, R...>::type;

class undefined;
// class undefined::foo; // impossibru

template <typename MemFoo>
struct specialize<typename defer_instantiation_t<undefined, MemFoo>::foo, MemFoo>
{
    void operator()(const MemFoo &f) const 
    {
        // this will compile however
        std::cout << f.name << std::endl;
    }
};

#include <string>

class undefined
{
private:
    struct foo{
        std::string name = "John Cena";
    };

    friend struct specialize<foo>; // this is irrelevant, but would be nicer than mem_foo

public:
    static foo get() { return {}; }
};

int main()
{
    specialize</*undefined::foo*/decltype(undefined::get())>{}(undefined::get());

    return 0;
}