Version compatibility issues with UNORDERED_MAP in C + + __c++

background

One of the most painful things in C + + is the unordered_map long-awaited, which is the only map in the earlier version of C + + standard library. But the internal implementation of the map is the use of red-black trees, as we all know, for dictionaries such structures can also be implemented with a hash table, that is, C + + standard library should also have hash_map this data structure. The red-black tree implements a map that takes up less memory, but looks inefficient, O (LOGN) lookup efficiency. The hash table implementation of the map occupies a large memory, but the search efficiency is high, often can approximate O (1) The amazing search efficiency.

The implementation of the hash table method for map in C + + is unordered_map this data structure, the first appearance should be in c++98 that era tr1 this namespace inside. Use the method to write more verbose.

#include <tr1/unordered_map>
using Std::tr1::unordered_map;

I think the data structure like UNORDERED_MAP is almost the daily necessities of C + + programmers. But because of the laziness of the C + + Standards Committee, it was dragged to the C++0X/C++11 standard to incorporate UNORDERED_MAP into STD standards. That is, only in the compiler that supports-std=c++0x and even-std=c++11 (for g++ It is probably g++4.4 this version to start supporting), you can use the following code unordered_map:

#include <unordered_map>
using Std::unordered_map;

Assuming that if all the world's g++ are already more than 4.4 version, there will be no problem with unordered_map, but the reality is brutal, always in some parts of the world server, still using g++-4.1.x, and for these servers to upgrade g++ is also very troublesome. All of our programs should consider compatibility with low-level versions. Solving Method

#if (__cplusplus = 201103L)
#include <unordered_map>
#include <unordered_set>
#else
# Include <tr1/unordered_map>
#include <tr1/unordered_set>
namespace std
{
    using std:: Tr1::unordered_map;
    Using Std::tr1::unordered_set;
}
#endif

The solution is to rely on __cplusplus the value of this macro in different C + + versions. For a compilation process that uses the-std=c++0x or-std=c++11 compilation options, the __cplusplus value is 201103L, otherwise it is a different value. Conclusions

The above method lowest probably can only support to c++98 this standard, because to c++98 only then began to have the TR1/UNORDERED_MAP, therefore your compiler if even c++98 all does not support, then obediently uses the ancient times the map which spreads to now. Reference http://www.cplusplus.com/forum/general/90675/http://stackoverflow.com/questions/2324658/ How-to-determine-the-version-of-the-c-standard-used-by-the-compiler