最糟糕的情况是使用普通的strcat(或sprintf),因为strcat接收C字符串,必须要"计数"才能找到结尾。对于长字符串来说,这会导致性能问题。C++风格的字符串更好,性能问题可能与内存分配有关,而不是计算长度。但是,字符串增长是几何级别的(每次需要增长时都会加倍),所以并不是那么可怕。
我非常怀疑所有上述方法最终的性能都相同或非常相似。如果有什么不同,我预计stringstream会更慢,因为它需要支持格式化的开销,但我也认为这是微不足道的。
由于这种东西很"有趣",我会用一个基准测试返回结果。
注:这些结果适用于我的机器,运行x86-64 Linux,使用g++ 4.6.3编译。其他操作系统、编译器和C++运行库实现可能会有所不同。如果性能对您的应用程序很重要,请在您关键的系统上使用您使用的编译器进行基准测试。以下是我编写的代码。它可能不是真实场景的完美表示,但我认为它是一个典型场景。
#include <iostream>
#include <iomanip>
#include <string>
#include <sstream>
#include <cstring>
using namespace std;
static __inline__ unsigned long long rdtsc(void)
{
unsigned hi, lo;
__asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi));
return ( (unsigned long long)lo)|( ((unsigned long long)hi)<<32 );
}
string build_string_1(const string &a, const string &b, const string &c)
{
string out = a + b + c;
return out;
}
string build_string_1a(const string &a, const string &b, const string &c)
{
string out;
out.resize(a.length()*3);
out = a + b + c;
return out;
}
string build_string_2(const string &a, const string &b, const string &c)
{
string out = a;
out += b;
out += c;
return out;
}
string build_string_3(const string &a, const string &b, const string &c)
{
string out;
out = a;
out.append(b);
out.append(c);
return out;
}
string build_string_4(const string &a, const string &b, const string &c)
{
stringstream ss;
ss << a << b << c;
return ss.str();
}
char *build_string_5(const char *a, const char *b, const char *c)
{
char* out = new char[strlen(a) * 3+1];
strcpy(out, a);
strcat(out, b);
strcat(out, c);
return out;
}
template<typename T>
size_t len(T s)
{
return s.length();
}
template<>
size_t len(char *s)
{
return strlen(s);
}
template<>
size_t len(const char *s)
{
return strlen(s);
}
void result(const char *name, unsigned long long t, const string& out)
{
cout << left << setw(22) << name << " time:" << right << setw(10) << t;
cout << " (per character: "
<< fixed << right << setw(8) << setprecision(2) << (double)t / len(out) << ")" << endl;
}
template<typename T>
void benchmark(const char name[], T (Func)(const T& a, const T& b, const T& c), const char *strings[])
{
unsigned long long t;
const T s1 = strings[0];
const T s2 = strings[1];
const T s3 = strings[2];
t = rdtsc();
T out = Func(s1, s2, s3);
t = rdtsc() - t;
if (len(out) != len(s1) + len(s2) + len(s3))
{
cout << "Error: out is different length from inputs" << endl;
cout << "Got `" << out << "` from `" << s1 << "` + `" << s2 << "` + `" << s3 << "`";
}
result(name, t, out);
}
void benchmark(const char name[], char* (Func)(const char* a, const char* b, const char* c),
const char *strings[])
{
unsigned long long t;
const char* s1 = strings[0];
const char* s2 = strings[1];
const char* s3 = strings[2];
t = rdtsc();
char *out = Func(s1, s2, s3);
t = rdtsc() - t;
if (len(out) != len(s1) + len(s2) + len(s3))
{
cout << "Error: out is different length from inputs" << endl;
cout << "Got `" << out << "` from `" << s1 << "` + `" << s2 << "` + `" << s3 << "`";
}
result(name, t, out);
delete [] out;
}
#define BM(func, size) benchmark(#func " " #size, func, strings ## _ ## size)
#define BM_LOT(size) BM(build_string_1, size); \
BM(build_string_1a, size); \
BM(build_string_2, size); \
BM(build_string_3, size); \
BM(build_string_4, size); \
BM(build_string_5, size);
int main()
{
const char *strings_small[] = { "Abc", "Def", "Ghi" };
const char *strings_medium[] = { "abcdefghijklmnopqrstuvwxyz",
"defghijklmnopqrstuvwxyzabc",
"ghijklmnopqrstuvwxyzabcdef" };
const char *strings_large[] =
{ "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"
"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz",
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc"
"defghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabc",
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
"ghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdef"
};
for(int i = 0; i < 5; i++)
{
BM_LOT(small);
BM_LOT(medium);
BM_LOT(large);
cout << "---------------------------------------------" << endl;
}
}
以下是一些代表性的结果:
build_string_1 small time: 4075 (per character: 452.78)
build_string_1a small time: 5384 (per character: 598.22)
build_string_2 small time: 2669 (per character: 296.56)
build_string_3 small time: 2427 (per character: 269.67)
build_string_4 small time: 19380 (per character: 2153.33)
build_string_5 small time: 6299 (per character: 699.89)
build_string_1 medium time: 3983 (per character: 51.06)
build_string_1a medium time: 6970 (per character: 89.36)
build_string_2 medium time: 4072 (per character: 52.21)
build_string_3 medium time: 4000 (per character: 51.28)
build_string_4 medium time: 19614 (per character: 251.46)
build_string_5 medium time: 6304 (per character: 80.82)
build_string_1 large time: 8491 (per character: 3.63)
build_string_1a large time: 9563 (per character: 4.09)
build_string_2 large time: 6154 (per character: 2.63)
build_string_3 large time: 5992 (per character: 2.56)
build_string_4 large time: 32450 (per character: 13.87)
build_string_5 large time: 15768 (per character: 6.74)
相同的代码,以32位运行:
build_string_1 small time: 4289 (per character: 476.56)
build_string_1a small time: 5967 (per character: 663.00)
build_string_2 small time: 3329 (per character: 369.89)
build_string_3 small time: 3047 (per character: 338.56)
build_string_4 small time: 22018 (per character: 2446.44)
build_string_5 small time: 3026 (per character: 336.22)
build_string_1 medium time: 4089 (per character: 52.42)
build_string_1a medium time: 8075 (per character: 103.53)
build_string_2 medium time: 4569 (per character: 58.58)
build_string_3 medium time: 4326 (per character: 55.46)
build_string_4 medium time: 22751 (per character: 291.68)
build_string_5 medium time: 2252 (per character: 28.87)
build_string_1 large time: 8695 (per character: 3.72)
build_string_1a large time: 12818 (per character: 5.48)
build_string_2 large time: 8202 (per character: 3.51)
build_string_3 large time: 8351 (per character: 3.57)
build_string_4 large time: 38250 (per character: 16.35)
build_string_5 large time: 8143 (per character: 3.48)
由此可得出以下结论:
最佳选项是逐个追加位(out.append()
或 out +=
),链式方法也相当不错。
预分配字符串并没有帮助。
使用 stringstream
是个相当糟糕的想法(速度慢了2-4倍)。
char *
使用了 new char[]
。在调用函数中使用局部变量可以使速度更快,但这种比较略微不公平。
组合短字符串有相当大的开销 - 只需复制数据最多每字节一个周期 [除非数据不适合缓存]。
编辑2
如评论所述,已添加:
string build_string_1b(const string &a, const string &b, const string &c)
{
return a + b + c;
}
并且。
string build_string_2a(const string &a, const string &b, const string &c)
{
string out;
out.reserve(a.length() * 3);
out += a;
out += b;
out += c;
return out;
}
这将产生以下结果:
build_string_1 small time: 3845 (per character: 427.22)
build_string_1b small time: 3165 (per character: 351.67)
build_string_2 small time: 3176 (per character: 352.89)
build_string_2a small time: 1904 (per character: 211.56)
build_string_1 large time: 9056 (per character: 3.87)
build_string_1b large time: 6414 (per character: 2.74)
build_string_2 large time: 6417 (per character: 2.74)
build_string_2a large time: 4179 (per character: 1.79)
(这个测试是在32位系统上进行的,但64位系统的结果非常相似。)
stringstream
是专门用于此场景的,而string
则不是。因此,最好从stringstream
开始尝试。 - Magnus Hoffl_czTempStr = std::string("Test data1") + "Test data2" + "Test data3";
是合法的,否则无法回答问题。除此之外,回答需要比较不同的技术所需的时间。由于涉及到许多变量,因此无法回答这个问题。答案取决于您要处理的字符串数量和长度,以及您编译和为其编译的平台。 - john+
会创建一个新对象,在C++11中有一些特殊情况)。但是,除非必要,否则不要对此进行优化,否则您的代码将难以阅读。 - Davestd::ostringstream
是用于格式化的,通常只在需要格式化输出时使用。所有的数据都是字符串,因此首选解决方案是使用std::string
和字符串拼接。 - James Kanze