Character Array as a value in C++ map

问题:

I want to define something like

Map<int, char[5] > myMap;

The above declaration is accepted by c++ compiler and no error is thrown but when I do something like this

int main()
{
    char arr[5] ="sdf";
    map <int, char[5]> myMap;
    myMap.insert(pair<int, char[5]>(0,arr));
    return 0;
}

I get error as:

In file included from /usr/include/c++/4.6/bits/stl_algobase.h:65:0,
                 from /usr/include/c++/4.6/bits/char_traits.h:41,
                 from /usr/include/c++/4.6/ios:41,
                 from /usr/include/c++/4.6/ostream:40,
                 from /usr/include/c++/4.6/iostream:40,
                 from charMap.cpp:1:
/usr/include/c++/4.6/bits/stl_pair.h: In constructor ‘std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = int, _T2 = char [5]]’:
charMap.cpp:9:42:   instantiated from here
/usr/include/c++/4.6/bits/stl_pair.h:104:31: error: array used as initializer
/usr/include/c++/4.6/bits/stl_pair.h: In constructor ‘std::pair<_T1, _T2>::pair(const std::pair<_U1, _U2>&) [with _U1 = int, _U2 = char [5], _T1 = const int, _T2 = char [5]]’:
charMap.cpp:9:43:   instantiated from here
/usr/include/c++/4.6/bits/stl_pair.h:109:39: error: array used as initializer

It is important for me to define a fixed size character array because it optimizes my network stream operation. Is there any way to achieve it? I do not want to use char * or std::string.

回答1:

One way is to wrap the fixed size character array as a struct.

struct FiveChar
{
   FiveChar(char in[5]) { memcpy(data, in, 5); }
   char& operator[](unsigned int idx) { return data[idx]; }
   char data[5];
};

int main(void)
{
   char arr[5] = "sdf";
   map<int, FiveChar> myMap;
   myMap.insert(pair<int, FiveChar>(0, arr));
   return 0;
}

回答2:

I understand your performance requirements (since I do similar things too), but using character arrays in that way is rather unsafe.

If you have access to C++11 you could use std::array. Then you could define your map like:

map <int, array<char, 5>> myMap;

If you cannot use C++11, then you could use boost::array.

回答3:

You cannot use an array in a standard container.

1. Use an std::vector instead of an array

2. Use a map of pointers to arrays of 5 elements.

3. Use boost tuples instead of arrays of 5 elements.

4. Instead of using an array make a new struct that takes 3 elements. Make the map<int, newstructtype>. Or wrap your array in a struct and that will work too.

\

struct ArrayMap
{
    int color[5];
};

/

回答4:

According to documentation pair can hold (and be defined for) distinct classes. C array is not a class, but a construct. You must define your own class with overloaded operator[](). Might need comparison operators as well