C++递归函数类型

Question

Rob Pike 在 2011 年 (链接) 发表了关于 go 中的词法分析器的演讲，他在那里定义了一个这样的类型：

// stateFn represents the state of the scanner
// as a function that returns the next state.
type stateFn func() stateFn

我想在 C++ 中实现相同的目标，但不知道如何：

// stateFn represents the state of the scanner
// as a function that returns the next state.
type stateFn func() stateFn

注意：这个问题可能有关联（在 Rust 中也是一样）

编辑：

这是我想要实现的目标：

// 01: error C3861: 'statefn_t': identifier not found
typedef std::function<statefn_t()> statefn_t;

// 02: error C2371: 'statefn_t': redefinition; different basic types
typedef std::function<class statefn_t()> statefn_t;

// 03: error C2371: 'statefn_t': redefinition; different basic types
typedef std::function<struct statefn_t()> statefn_t;

// 04: error C2065: 'statefn_t': undeclared identifier
typedef std::function<statefn_t*()> statefn_t;

// 05: error C2371: 'statefn_t': redefinition; different basic types
typedef std::function<class statefn_t*()> statefn_t;

// 06: error C2371: 'statefn_t': redefinition; different basic types
typedef std::function<struct statefn_t*()> statefn_t;

Answer 1

类型别名不能递归。

要实现像 go 讲座中使用的那样的状态机，您需要定义一个自定义类型：

class state
{
public:
    using fn = std::function<state()>;
    state() {}
    state(fn f) : f(f){}
    operator bool() { return (bool)f; }
    operator fn () { return f; }

private:
    fn f;
};

用法：

state::fn stateEnd()
{
    std::cout << "end\n";
    return {};
}
state::fn stateTransit()
{
    std::cout << "transit\n";
    return stateEnd;
}

state::fn stateStart()
{
    std::cout << "start\n";
    return stateTransit;
}


int main() {
    state::fn s = stateStart;
    while(s = s());
}

替代形式：

class state
{
public:
    state() {}
    template<class T>
    state(T&& t) : f(std::forward<T>(t)){}
    operator bool() { return (bool)f; }
    state operator()() { return f(); }

private:
    std::function<state()> f;
};

用法：

state stateEnd()
{
    std::cout << "end\n";
    return {};
}
state stateTransit()
{
    std::cout << "transit\n";
    return stateEnd;
}

state stateStart()
{
    std::cout << "start\n";
    return stateTransit;
}


int main() {
    state s {stateStart};
    while(s = s());
}