是`boost::function<int(const char*)> f` 是指针还是 object？</int(const>

Question

As per the article( https://theboostcpplibraries.com/boost.function ), which says that:

boost::function makes it possible to define a pointer to a function with a specific signature. boost::function<int(const char*)> f defines a pointer f that can point to functions that expect a parameter of type const char* and return a value of type int.

How to comprehend that boost::function<int(const char*)> f defines a pointer f that can point to a function.

Doesn't f is an object with the type of boost::function<int(const char*)> , it's not a pointer indeed.

Answer 1

@sehe Sorry, I forgot. The link is added to the post. theboostcpplibraries.com/boost.function – John 37 mins ago

Thanks for the added link.

Though I have his book in print and value his contributions to Boost introductory materials, I stand by my assessment that the wording here is unnecessarily confusing, and even misguiding: it seems to completely miss the fact that non-function callables can be assigned. This, IMO, is the whole point of doing the type erasure.

If you just wanted to define function pointers easily, you could just use function pointers:

static int my_function(const char*) { return 42; }

// regular function pointer stuff
{
    using MyFunction = int(const char*);
    MyFunction* pf = &my_function;

    if (pf) {
        std::cout << "pf(): " << pf(argument) << "\n";
    }
}

But with boost::function (or std::function ) you can also do:

// gereralized assignable callables:
boost::function<int(const char*)> f = &my_function;
if (f) {
    std::cout << "same with f(): " << f(argument) << "\n";
}
struct MyCallable {
    int operator()(const char*) const { return _retval; }
    int _retval = 42;
};

f = MyCallable{};
std::cout << f(argument) << "\n"; // prints 42

f = MyCallable{99};
std::cout << f(argument) << "\n"; // prints 99

MyCallable instance { 10 };
f = std::ref(instance);
while (instance._retval--)
    std::cout << f(argument) << "\n"; // prints 9 .. 0

LIVE DEMO

Live On Coliru

#include <boost/function.hpp>
#include <iostream>

static int my_function(const char*) { return 42; }

int main() {
    auto argument = "dummy";
    // regular function pointer stuff
    {
        using MyFunction = int(const char*);
        MyFunction* pf = &my_function;

        if (pf) {
            std::cout << "pf(): " << pf(argument) << "\n";
        }
    }

    // gereralized assignable callables:
    boost::function<int(const char*)> f = &my_function;
    if (f) {
        std::cout << "same with f(): " << f(argument) << "\n";
    }
    struct MyCallable {
        int operator()(const char*) const { return _retval; }
        int _retval = 42;
    };

    f = MyCallable{};
    std::cout << f(argument) << "\n"; // prints 42

    f = MyCallable{99};
    std::cout << f(argument) << "\n"; // prints 99

    MyCallable instance { 10 };
    f = std::ref(instance);
    while (instance._retval--)
        std::cout << f(argument) << "\n"; // prints 9 .. 0
}

Prints

pf(): 42
same with f(): 42
42
99
9
8
7
6
5
4
3
2
1
0