今天我谈论的是 C++11 中的新闻功能,如线程、to_string 和 stoi。
但事实上,这一切在 C++98 中都已经成为可能。
然后我决定比较旧库与新闻库:
C++11:
#include <iostream>
#include <string>
#include <ctime>
using namespace std;
int main()
{
clock_t tStart = clock();
string input = "50";
for (int i = 0; i < 50000; i++)
{
int number = stoi(input);
}
cout << (double)(clock() - tStart) << endl;
return 0;
}
C++98
#include <iostream>
#include <string>
#include <sstream>
#include <ctime>
using namespace std;
int main()
{
clock_t tStart = clock();
string input = "50";
stringstream ss;
for (int i = 0; i < 50000; i++)
{
int number;
ss << input;
ss >> number;
}
cout << (double)(clock() - tStart) << endl;
return 0;
}
屏幕截图:
Windows 7 轻型服务器:
Ubuntu 14.04:
Slackware Server 14.1 通过 SSH:
优化
Windows 7 轻型服务器:
Ubuntu 14.04:
Slackware Server 14.1 通过 SSH:
配置:
Windows 7 轻型服务器:Intel(R) Core(TM) i5-4590 CPU @ 3.30GHz
Ubuntu 14.04:Intel(R) Core(TM) i7-2670QM CPU @ 2.20GHz
Slackware Server 14.1 通过 SSH:Intel(R) Core(TM) i3-4150 CPU @ 3.50GHz
问题:
stoi 和 stringstream 有什么区别?
为什么以及何时在字符串库(stoi,to_string)中使用新闻函数?
最后...... stoi 使用 -O3 快得多,而 sstream 则不然。那么,与 sstream 相比,-O3 对 stoi 的作用是什么?
最佳答案
我刚刚检查了以下内容:
gcc -v:
Using built-in specs.
COLLECT_GCC=x86_64-alt-linux-gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-alt-linux/5/lto-wrapper
Target: x86_64-alt-linux
Configured with: ../configure --host=x86_64-alt-linux --build=x86_64-alt-linux --target=x86_64-alt-linux --prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64 --libexecdir=/usr/libexec --localstatedir=/var/lib --sharedstatedir=/var/lib --mandir=/usr/share/man --infodir=/usr/share/info --disable-dependency-tracking --without-included-gettext --enable-shared --program-suffix=-5 --with-slibdir=/lib64 --with-bugurl=http://bugzilla.altlinux.org --enable-__cxa_atexit --enable-threads=posix --enable-checking=release --with-system-zlib --without-included-gettext --enable-multilib --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --with-arch_32=i586 --with-tune_32=generic --with-multilib-list=m64,m32,mx32 --enable-bootstrap --enable-languages=c,c++,fortran,objc,obj-c++,java,ada,go,lto --enable-plugin --enable-java-awt=gtk --with-native-libdir=/usr/lib64/gcj-5 --with-ecj-jar=/usr/share/java/ecj.jar --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-1.5.0.0/jre --enable-libgcj-multifile --disable-libjava-multilib --enable-java-maintainer-mode
Thread model: posix
gcc version 5.3.1 20151207 (ALT Linux 5.3.1-alt1) (GCC)
编码:
#include <iostream>
#include <string>
#include <chrono>
#include <sstream>
using namespace std;
using namespace chrono;
void stringstream_test(const string& input, int repetitions) {
stringstream ss;
auto start = steady_clock::now();
for (int i = 0; i < repetitions; i++) {
int number;
ss << input;
ss >> number;
}
auto end = steady_clock::now();
auto dur = duration_cast<nanoseconds>(end - start);
cout << "stringstream_test completed in " << dur.count() << " nanoseconds." << endl;
}
void stoi_test(const string& input, int repetitions) {
auto start = steady_clock::now();
for (int i = 0; i < repetitions; i++) {
int number = stoi(input);
}
auto end = steady_clock::now();
auto dur = duration_cast<nanoseconds>(end - start);
cout << "stoi_test completed in " << dur.count() << " nanoseconds." << endl;
}
int main() {
stringstream_test("50", 500000);
stoi_test("50", 500000);
return 0;
}
编译为
c++ -std=c++11 -O3 -o stoi_perf stoi_perf.cc
。结果,使用
while true; do ./stoi_perf; done
接收并在重复几次后停止:stringstream_test completed in 10449080 nanoseconds.
stoi_test completed in 10437559 nanoseconds.
stringstream_test completed in 9074869 nanoseconds.
stoi_test completed in 9895661 nanoseconds.
stringstream_test completed in 11516788 nanoseconds.
stoi_test completed in 12266627 nanoseconds.
stringstream_test completed in 10017085 nanoseconds.
stoi_test completed in 9468441 nanoseconds.
stringstream_test completed in 9957401 nanoseconds.
stoi_test completed in 9483004 nanoseconds.
stringstream_test completed in 10069845 nanoseconds.
stoi_test completed in 9657653 nanoseconds.
stringstream_test completed in 9568359 nanoseconds.
stoi_test completed in 9162406 nanoseconds.
stringstream_test completed in 9868536 nanoseconds.
stoi_test completed in 9166439 nanoseconds.
stringstream_test completed in 9059762 nanoseconds.
stoi_test completed in 11558076 nanoseconds.
stringstream_test completed in 11673084 nanoseconds.
stoi_test completed in 13432386 nanoseconds.
...
正如您可能看到的,两种测试都给出了可比的结果,有时一种比另一种略好。我的工作站很忙(有一个后台进程完全占用了六个内核中的一两个),但即使在这种情况下,测试结果对我来说还是令人满意的。
关于c++ - 为什么 stoi 比没有 -O3 的 stringstream 慢得多?,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/35307176/