C/C++&Java 字符串拼接效率对比
C/C++&Java 字符串拼接效率对比
C/C++拼接字符串的方式很多，尤其是所谓的”VC++”语言（不应该称其为语言，但是很多使用VC++的程序员却把它变成了VC++语言即微软血统的C++语言）中这种方式又变多一些。在Windows下的Visual C++中很多的程序员习惯使用微软MFC中CString 。虽然使用MFC编程时使用CString来处理字符串的确有着与MFC库配合的便利。但是在开发中如果并没有用到MFC的时候我建议还使用 Standard C++的字符串。好处很多首先它是标准的意味着你使用它（std::string, std::wstring）来处理字符串。并且你的程序没有使用与具体操作系统相关的API （或使用了跨平台的相关类库代替）。 那么意味着你的代码在有对应平台的并且符合C++标准的编译器下可以直接编译通过。也意味着你的程序具备较好的跨平台能力。另外在性能方面也较为出色。下面测试了一下在VC++2008中各种字符串拼接方式的性能对比。当然这里面有有预分配方式的，测试程序中最后会根据每一项的耗时进行排序，当然排序的结果需要看是否采用预分配方式。当然C++ std::string, std::wstring中有一些提高效率的机制（在非预分配方式下）。然后又在Java中测试了一下字符Java的StringBuffer及StringBuilder字符串拼接效率也很不错。最后测试的结果有助于指导我们使用最高效的方式处理字符处拼接。
字符串拼接测试方式分别是:
1.C语言在预分配内存上的strcpy函数
2.C语言在预分配内存上的 memcpy函数
3.C++中std::string的 append函数
4.C++中std::string 的 operator+=
5.C++中std::ostringstream的 operator<<
6.C++中std::string使用事先预分配（调用 reserve函数）
7.MFC中CString的Append函数
8.MFC中CString的operator+=
以下测试代码：
注：在 const int TEST_TIMES = 9000000;时C的memcpy使用16-32 ms(毫秒) std::string 也在 1000 ms(毫秒)左右。由于其他的方式耗时过长因此将 TEST_TIMES = 90000次 在Java中普通的String也是如此.
#include "stdafx.h"#include <string>#include <sstream>#include <iostream>#include <cassert>#include <cstring>#include <ctime>#include <vector>#include <algorithm>#include <afxwin.h>using namespace std;const int TEST_TIMES = 90000;const char APPEND_CONTENT[] = "cppmule";const int PREALLOCATE_SIZE = strlen(APPEND_CONTENT) * TEST_TIMES + 1;class Statistic {public: string item; int used; friend inline ostream & operator << (ostream & os, const Statistic &stat) { os << "item: " << stat.item << endl << "used: " << stat.used << " ms." << endl << endl; return os; } inline bool operator>(const Statistic& stat) { return (used > stat.used); } inline bool operator<(const Statistic& stat) { return (used < stat.used); }};vector<Statistic> g_statistics;#define BEGIN_TICK() \ clock_t start = clock();#define END_AND_PRINT_TICK(info) \ clock_t used = clock() - start; \ Statistic stat; \ stat.item.assign(info); \ stat.used = used; \ g_statistics.push_back(stat); \ cout << info << " Used: " << used << " ms." << endl;#define PRINT_SORT_TEST_TICKS() \ sort(g_statistics.begin(), g_statistics.end()); \ struct StatisticPrinter { \ StatisticPrinter() : order(0) {} \ void operator() (const Statistic& stat) { \ ++order; \ cout << "sort order: " << order << endl \ << stat; \ } \ int order; \ } printer; \cout << "---------Statistics informations(sorting ascendent)-------" << endl << endl; \ for_each(g_statistics.begin(), g_statistics.end(), printer);\ cout << "----------------------" << endl;void test_stdstring_append(){ string str; BEGIN_TICK(); for (int i=0; i<TEST_TIMES; i++) { str.append(APPEND_CONTENT); } END_AND_PRINT_TICK("std::string append"); cout << "string length: " << str.length() << endl;}void test_stdstring_append_operator(){ string str; BEGIN_TICK(); for (int i=0; i<TEST_TIMES; i++) { str += APPEND_CONTENT; } END_AND_PRINT_TICK("std::string += operator"); cout << "string length: " << str.length() << endl;}void test_stdostringstream(){ ostringstream oss; BEGIN_TICK(); for (int i=0; i<TEST_TIMES; i++) { oss << APPEND_CONTENT; } END_AND_PRINT_TICK("std::ostringstream <<"); cout << "string length: " << oss.str().length() << endl;}void test_stdostringstream_preallocate(){ ostringstream oss; BEGIN_TICK(); for (int i=0; i<TEST_TIMES; i++) { oss << APPEND_CONTENT; } END_AND_PRINT_TICK("std::ostringstream <<"); cout << "string length: " << oss.str().length() << endl;}void test_stdstring_append_operator_preallocate(){ string str; str.reserve(PREALLOCATE_SIZE); cout << "capacity: " << str.capacity() << endl << "size: " << str.size() << endl; //assert(str.capacity() == 1024*1024*512); BEGIN_TICK(); for (int i=0; i<TEST_TIMES; i++) { str += APPEND_CONTENT; } END_AND_PRINT_TICK("hava resize(PREALLOCATE_SIZE) std::string += operator"); cout << "string length: " << str.length() << endl;}void test_c_strcat_append(){ char* pstr = (char*)malloc(PREALLOCATE_SIZE); memset(pstr, 0, sizeof(pstr)); BEGIN_TICK(); for (int i=0; i<TEST_TIMES; i++) { strcat(pstr, APPEND_CONTENT); } END_AND_PRINT_TICK("c string function strcat:"); cout << "string length: " << strlen(pstr) << endl; free(pstr); pstr = NULL;}void test_c_memcpy_append(){ //Allocate memory char* pstr = (char*)malloc(PREALLOCATE_SIZE); if (NULL == pstr) { cerr << "Can't allocate memory." << endl; return; } memset(pstr, 0, PREALLOCATE_SIZE); BEGIN_TICK(); int len = 0; for (int i=0; i<TEST_TIMES; i++) { memcpy(pstr + len, APPEND_CONTENT, strlen(APPEND_CONTENT)); len += strlen(APPEND_CONTENT); } END_AND_PRINT_TICK("C language memcpy append"); cout << "string length: " << strlen(pstr) << endl; //Cleanup free(pstr); pstr = NULL;}void test_mfc_cstring_append(){ CString str; BEGIN_TICK(); for (int i=0; i<TEST_TIMES; i++) { str.Append(APPEND_CONTENT); } END_AND_PRINT_TICK("MFC CString append"); cout << "string length: " << str.GetLength() << endl;}void test_mfc_cstring_append_operator(){ CString str; BEGIN_TICK(); for (int i=0; i<TEST_TIMES; i++) { str += APPEND_CONTENT; } END_AND_PRINT_TICK("MFC CString operator append"); cout << "string length: " << str.GetLength() << endl;}int _tmain(int argc, _TCHAR* argv[]){#ifdef _DEBUG cout << "DEBUG version." << endl;#else cout << "Release version." << endl;#endif cout << "TEST_TIME: " << TEST_TIMES << endl; test_c_memcpy_append(); test_stdstring_append_operator(); test_stdstring_append(); test_stdostringstream(); test_stdstring_append_operator_preallocate(); test_mfc_cstring_append(); test_mfc_cstring_append_operator(); test_c_strcat_append(); PRINT_SORT_TEST_TICKS(); return 0;}
void testJavaStringPerformance() {final int TEST_TIMES = 90000; //String String str = new String(); System.out.println("The testing is running, please wait..."); long start = System.currentTimeMillis(); for (int i=0; i<TEST_TIMES; i++) { str += "cppmule"; } long strUsed = System.currentTimeMillis() - start; System.out.println("strUsed: " + strUsed + " ms."); //StringBuffer start = 0; StringBuffer strBuffer = new StringBuffer(); start = System.currentTimeMillis(); for (int i=0; i<TEST_TIMES; i++) { strBuffer.append("cppmule"); } long strBufferUsed = System.currentTimeMillis() - start; System.out.println("StringBuffer append: " + strBufferUsed + " ms."); //StringBuilder start = 0; StringBuilder strBuilder = new StringBuilder(); start = System.currentTimeMillis(); for (int i=0; i<TEST_TIMES; i++) { strBuilder.append("cppmule"); } long strBuilderUsed = System.currentTimeMillis() - start; System.out.println("StringBuilder append: " + strBuilderUsed + " ms."); System.out.println("Times: " + TEST_TIMES);}