插入stl :: map时出现问题

Question

I am trying to create a <function name, function ptr> pair using std::map

typedef int(*FnPtr)()

int Foo()
{
    return 1;
}

map<const char*, FnPtr> funcMap;
funcMap.insert("Foo", Foo);        // Error

I get the following error message:

Error: no instance of overloaded function "std:map<_Kty, _Ty, _Pr, _Alloc>::insert[with _Kty=const char*, _Ty=FnPtr,_Pr=std::less < const char * > , _Alloc=std::allocator< std::pair< const char* const, FnPtr>>]" matches the argument list

argument types are: (const char[3], void())

object type is:

std::map< const char*, FnPtr, std::less < const char *>, std::allocator< std::pair < const char* const, FnPtr>>>

Does std:map not support custom types?

Answer 1

map::insert takes an std::pair as argument, so you need

funcMap.insert(std::make_pair("Foo", Foo));

This will fix the compilation error, but, assuming you care about the ordering of elements, it's unlikely your map will behave as you expect it to. As defined, your keys are going to have arbitrary ordering depending upon the address that the string literals happen to be placed in memory by the compiler because the map comparator is going to compare addresses of the string literals, and not their contents.

If you want the keys to be ordered by the string contents, then the easiest way is to change the map to

std::map<std::string, FnPtr> funcMap;

---

If you want to avoid using std::string , and work with string literals only, then you can define a custom comparator for comparing the strings

#include <cstring>

struct str_literal_less
{
    bool operator()(char const *l, char const *r) const
    { return std::strcmp(l, r) < 0; }
};

Then define the map as

std::map<char const *, FnPtr, str_literal_less> funcMap;

Answer 2

You have to insert a pair into a map. Try:

funcMap.insert(make_pair("Foo", Foo));

If you have C++11 support, you can do

funcMap.insert({"Foo", Foo});

Alternatively, why can't you just do this?

funcMap["Foo"] = Foo;

Answer 3

You have to change

funcMap.insert("Foo", Foo);

In

funcMap.insert(make_pair("Foo", Foo));

For more info you can read here http://www.cplusplus.com/reference/map/map/insert/