函数ToString()使用std::string::reserve()来实现最小化再分配。下面你可以看到一个性能测试的结果。
函数join()使用std::accumulate()，和一个已经为首个操作数预留内存的自定义函数。
你可能会问，为什么StringBuilder::m_Data用std::list而不是std::vector？除非你有一个用其他容器的好理由，通常都是使用std::vector。
好吧，我（这样做）有两个原因：
1. 字符串总是会附加到一个容器的末尾。std::list允许在不需要内存再分配的情况下这样做；因为vector是使用一个连续的内存块实现的，每用一个就可能导致内存再分配。
2. std::list对顺序存取相当有利，而且在m_Data上所做的唯一存取操作也是顺序的。
你可以建议同时测试这两种实现的性能和内存占用情况，然后选择其中一个。
性能评估
为了测试性能，我从Wikipedia获取一个网页，并将其中一部分内容写死到一个string的vector中。
随后，我编写两个测试函数，第一个在两个循环中使用标准函数clock()并调用std::accumulate()和StringBuilder::ToString()，然后打印结果。
void TestPerformance(const StringBuilder<wchar_t> &tested, const std::vector<std::wstring> &tested2) {
const int loops = 500;
clock_t start = clock(); // Give up some accuracy in exchange for platform independence.
for (int i = 0; i < loops; ++i) {
std::wstring accumulator;
std::accumulate(tested2.begin(), tested2.end(), accumulator);
}
double secsAccumulate = (double) (clock() - start) / CLOCKS_PER_SEC;
start = clock();
for (int i = 0; i < loops; ++i) {
std::wstring result2 = tested.ToString();
}
double secsBuilder = (double) (clock() - start) / CLOCKS_PER_SEC;
using std::cout;
using std::endl;
cout << "Accumulate took " << secsAccumulate << " seconds, and ToString() took " << secsBuilder << " seconds."
<< " The relative speed improvement was " << ((secsAccumulate / secsBuilder) - 1) * 100 << "%"
<< endl;
}
第二个则使用更精确的Posix函数clock_gettime()，并测试StringBuilder::Join()。
#ifdef __USE_POSIX199309
// Thanks to <a href="http://www.easck.com/2007/09/22/profiling-code-using-clock_gettime/">Guy Rutenberg</a>.
timespec diff(timespec start, timespec end)
{
timespec temp;
if ((end.tv_nsec-start.tv_nsec)<0) {
temp.tv_sec = end.tv_sec-start.tv_sec-1;
temp.tv_nsec = 1000000000+end.tv_nsec-start.tv_nsec;
} else {
temp.tv_sec = end.tv_sec-start.tv_sec;
temp.tv_nsec = end.tv_nsec-start.tv_nsec;
}
return temp;
}
void AccurateTestPerformance(const StringBuilder<wchar_t> &tested, const std::vector<std::wstring> &tested2) {
const int loops = 500;
timespec time1, time2;
// Don't forget to add -lrt to the g++ linker command line.
////////////////
// Test std::accumulate()
////////////////
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &time1);
for (int i = 0; i < loops; ++i) {
std::wstring accumulator;
std::accumulate(tested2.begin(), tested2.end(), accumulator);
}
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &time2);
using std::cout;
using std::endl;
timespec tsAccumulate =diff(time1,time2);
cout << tsAccumulate.tv_sec << ":" << tsAccumulate.tv_nsec << endl;
////////////////
// Test ToString()
////////////////
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &time1);
for (int i = 0; i < loops; ++i) {
std::wstring result2 = tested.ToString();
}
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &time2);
timespec tsToString =diff(time1,time2);
cout << tsToString.tv_sec << ":" << tsToString.tv_nsec << endl;
////////////////
// Test join()
////////////////
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &time1);
for (int i = 0; i < loops; ++i) {
std::wstring result3 = tested.Join(L",");
}
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &time2);
timespec tsJoin =diff(time1,time2);
cout << tsJoin.tv_sec << ":" << tsJoin.tv_nsec << endl;
////////////////
// Show results
////////////////
double secsAccumulate = tsAccumulate.tv_sec + tsAccumulate.tv_nsec / 1000000000.0;
double secsBuilder = tsToString.tv_sec + tsToString.tv_nsec / 1000000000.0;
double secsJoin = tsJoin.tv_sec + tsJoin.tv_nsec / 1000000000.0;
cout << "Accurate performance test:" << endl << " Accumulate took " << secsAccumulate << " seconds, and ToString() took " << secsBuilder << " seconds." << endl
<< " The relative speed improvement was " << ((secsAccumulate / secsBuilder) - 1) * 100 << "%" << endl <<
" Join took " << secsJoin << " seconds."
<< endl;
}
#endif // def __USE_POSIX199309
