在閱讀代碼時，發現有使用size()==0判斷是否容器爲空的，而從<<Effective STL>>上看到size()不能保證常數時間，建議使用empty()替換。因此我做了一個實驗，發現size()並不能保證常數時間，但empty可以保證。
/**
測試vector、list、set調用empty和size的耗時是否爲常數，
結論：empty()的調用時間都是常數，list的size()的調用時間非常數
使用建議：判斷成員是否爲空時使用empty()，而非size() == 0
input COUNT:100000(10W) 1000000(100W)
output
member count is:1000000
test vector.empty():
cost time(ms):0
test vector.size():
cost time(ms):0
---------------------
test list.empty():
cost time(ms):0
test list.size():
cost time(ms):8
---------------------
test set.empty():
cost time(ms):0
test set.size():
cost time(ms):0
---------------------
member count is:10000000
test vector.empty():
cost time(ms):0
test vector.size():
cost time(ms):0
---------------------
test list.empty():
cost time(ms):0
test list.size():
cost time(ms):79
---------------------
test set.empty():
cost time(ms):0
test set.size():
cost time(ms):0
---------------------
*/
#include <iostream>
#include <vector>
#include <set>
#include <list>
#include <sys/time.h>
using namespace std;
typedef unsigned long long ull;
#define COST_TIME_START \
{\
timeval start; \
gettimeofday(&start, NULL);
#define COST_TIME_END \
timeval end;\
gettimeofday(&end, NULL); \
cout << "cost time(ms):" << ((end.tv_sec*1000 + end.tv_usec/1000) - (start.tv_sec*1000 + start.tv_usec/1000)) << endl; \
}
int main (int argc, char **argv) {
cout << "member count is:" << COUNT << endl;
vector<ull> v;
v.assign(COUNT, 0);
cout << "test vector.empty():" << endl;
COST_TIME_START
v.empty();
COST_TIME_END
cout << "test vector.size():" << endl;
COST_TIME_START
v.size();
COST_TIME_END
cout << "---------------------" << endl;
list<ull> l;
l.assign(COUNT, 0);
cout << "test list.empty():" << endl;
COST_TIME_START;
l.empty();
COST_TIME_END;
cout << "test list.size():" << endl;
COST_TIME_START;
l.size();
COST_TIME_END;
cout << "---------------------" << endl;
set<ull> s;
for (ull i = 0; i < COUNT; ++i) {
s.insert(i);
}
cout << "test set.empty():" << endl;
COST_TIME_START
s.empty();
COST_TIME_END
cout << "test set.size():" << endl;
COST_TIME_START
s.size();
COST_TIME_END
cout << "---------------------" << endl;
return 0;
}