printf()和cout在c++中的区别是什么?
当前回答
我想指出的是,如果你想在c++中使用线程,如果你使用cout,你可以得到一些有趣的结果。
考虑下面的代码:
#include <string>
#include <iostream>
#include <thread>
using namespace std;
void task(int taskNum, string msg) {
for (int i = 0; i < 5; ++i) {
cout << "#" << taskNum << ": " << msg << endl;
}
}
int main() {
thread t1(task, 1, "AAA");
thread t2(task, 2, "BBB");
t1.join();
t2.join();
return 0;
}
// g++ ./thread.cpp -o thread.out -ansi -pedantic -pthread -std=c++0x
现在,输出都被打乱了。它也可以产生不同的结果,试着执行几次:
##12:: ABABAB
##12:: ABABAB
##12:: ABABAB
##12:: ABABAB
##12:: ABABAB
你可以使用printf,也可以使用互斥。
#1: AAA
#2: BBB
#1: AAA
#2: BBB
#1: AAA
#2: BBB
#1: AAA
#2: BBB
#1: AAA
#2: BBB
玩得开心!
其他回答
TL;DR:在相信在线随机评论之前,一定要自己做研究,考虑生成的机器代码的大小、性能、可读性和编码时间,包括这一条。
我不是专家。我碰巧听到两个同事在讨论如何避免在嵌入式系统中使用c++,因为会导致性能问题。有趣的是,我基于一个真实的项目任务做了一个基准测试。
在该任务中,我们必须向RAM写入一些配置。喜欢的东西:
咖啡=热 糖=没有 牛奶=乳房 mac = AA: BB: CC:弟弟:EE: FF
这是我的基准测试程序(是的,我知道OP询问printf(),而不是fprintf()。试着捕捉本质,顺便说一下,OP的链接指向fprintf()。)
C程序:
char coffee[10], sugar[10], milk[10];
unsigned char mac[6];
/* Initialize those things here. */
FILE * f = fopen("a.txt", "wt");
fprintf(f, "coffee=%s\nsugar=%s\nmilk=%s\nmac=%02X:%02X:%02X:%02X:%02X:%02X\n", coffee, sugar, milk, mac[0], mac[1],mac[2],mac[3],mac[4],mac[5]);
fclose(f);
c++程序:
//Everything else is identical except:
std::ofstream f("a.txt", std::ios::out);
f << "coffee=" << coffee << "\n";
f << "sugar=" << sugar << "\n";
f << "milk=" << milk << "\n";
f << "mac=" << (int)mac[0] << ":"
<< (int)mac[1] << ":"
<< (int)mac[2] << ":"
<< (int)mac[3] << ":"
<< (int)mac[4] << ":"
<< (int)mac[5] << endl;
f.close();
我尽了最大努力打磨它们,然后把它们都绕了10万次。以下是调查结果:
C程序:
real 0m 8.01s
user 0m 2.37s
sys 0m 5.58s
c++程序:
real 0m 6.07s
user 0m 3.18s
sys 0m 2.84s
目标文件大小:
C - 2,092 bytes
C++ - 3,272 bytes
结论:在我非常特定的平台上,使用非常特定的处理器,运行非常特定版本的Linux内核,运行一个非常特定版本的GCC编译的程序,以完成一个非常特定的任务,我会说c++方法更适合,因为它运行得更快,可读性更好。另一方面,C提供了小的内存占用,在我看来,这几乎没有什么意义,因为程序大小不是我们所关心的。
记住,YMMV。
更多的差异: "printf"返回一个整数值(等于打印的字符数),"cout"不返回任何东西
And.
Cout << "y = " << 7;不是原子的。
Printf ("%s = %d", "y", 7);是原子的。
Cout执行类型检查,printf不执行。
iostream中没有% d的等价物
来自c++常见问题解答:
[15.1] Why should I use <iostream> instead of the traditional <cstdio>? Increase type safety, reduce errors, allow extensibility, and provide inheritability. printf() is arguably not broken, and scanf() is perhaps livable despite being error prone, however both are limited with respect to what C++ I/O can do. C++ I/O (using << and >>) is, relative to C (using printf() and scanf()): More type-safe: With <iostream>, the type of object being I/O'd is known statically by the compiler. In contrast, <cstdio> uses "%" fields to figure out the types dynamically. Less error prone: With <iostream>, there are no redundant "%" tokens that have to be consistent with the actual objects being I/O'd. Removing redundancy removes a class of errors. Extensible: The C++ <iostream> mechanism allows new user-defined types to be I/O'd without breaking existing code. Imagine the chaos if everyone was simultaneously adding new incompatible "%" fields to printf() and scanf()?! Inheritable: The C++ <iostream> mechanism is built from real classes such as std::ostream and std::istream. Unlike <cstdio>'s FILE*, these are real classes and hence inheritable. This means you can have other user-defined things that look and act like streams, yet that do whatever strange and wonderful things you want. You automatically get to use the zillions of lines of I/O code written by users you don't even know, and they don't need to know about your "extended stream" class.
另一方面,printf要快得多,因此在非常特定和有限的情况下,可以优先使用它而不是cout。总是先做侧写。(例如,参见http://programming-designs.com/2009/02/c-speed-test-part-2-printf-vs-cout/)
cout<< "Hello";
printf("%s", "Hello");
两者都用于打印值。它们有完全不同的语法。c++两者都有,C 只有printf。
这里没有提到的两点我认为很重要:
1)如果你还没有使用STL, cout会携带很多包袱。它向目标文件中添加的代码是printf的两倍多。对于字符串也是如此,这也是我倾向于使用自己的字符串库的主要原因。
2) cout使用重载的<<操作符,我觉得这很不幸。如果还将<<运算符用于其预期目的(左移),则会增加混淆。我个人不喜欢为了与其预期用途无关的目的而重载操作符。
底线:如果我已经在使用STL,我将使用cout(和字符串)。否则,我倾向于避免它。