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cout slowest processor MPI

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问题 I am writing a program using MPI. Each processor executes a for loop: int main(int argc, char** argv) { boost::mpi::environment env(argc, argv); for( int i=0; i<10; ++i ) { std::cout << "Index " << i << std::endl << std::flush; } } Is there a way to make the cout only happen on the last processor to hit index i? Or flag so a line is only executed on the last processor to get to it? 回答1: It might look like trivial, but actually, what you ask here is extremely complex for distributed memory

C++ cout printing slowly

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I noticed if I print out a long string(char*) using cout it seems to print 1 character at a time to the screen in Windows 7, Vista, and Linux(using putty) using Visual C++ 2008 on Windows and G++ on Linux. Printf is so much faster I actually switched from cout to printf for most printing in a project of mine. This is confusing me because this question makes it seem like I'm the only one having this issue. I even wrote a cout replacement that looks like it beats the pants off of cout on my comp - class rcout { public: char buff[4096]; unsigned int size; unsigned int length; rcout() { size =

What is the difference between cout, cerr, clog of iostream header in c++? When to use which one?

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问题 I tried researching the difference between cout , cerr and clog on the internet but couldn't find a perfect answer. I still am not clear on when to use which. Can anyone explain to me, through simple programs and illustrate a perfect situation on when to use which one? I visited this site which shows a small program on cerr and clog , but the output obtained over there can also be obtained using cout . So, I'm confused over each one's exact use. 回答1: stdout and stderr are different streams,

Why does this code produce a warning referring to the comma operator?

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问题 When answering this question, I came across this code... #include <iostream> int main() { int const income = 0; std::cout << "I'm sorry your income is: " < income; // this is line 6 } ...which contains a typo. The second (intended) << operator on line 6 has been accidentally written as a < . That aside, compiling the code using GCC 4.3.4 or 4.4.3 results in a warning: prog.cpp: In function ‘int main()’: prog.cpp:6: warning: right-hand operand of comma has no effect My question: why is that

C++ SegFault when dereferencing a pointer for cout

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问题 I'm new to C++ and just trying to get a hang of it. It generally seems not too bad, but I stumbled upon this weird/pathological segfaulting behavior: int main () { int* b; *b = 27; int c = *b; cout << "c points to " << c << endl; //OK printf( "b points to %d\n", *b); //OK // cout << "b points to " << (*b) << endl; - Not OK: segfaults! return 0; } This program, as given, produces what you'd expect: c points to 27 b points to 27 On the other hand, if you uncomment the second-to-last line, you

What's the difference between cout<<cout and cout<<&cout in c++?

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问题 This might be a beginner question and understanding how cout works is probably key here. If somebody could link to a good explanation, it would be great. cout<<cout and cout<<&cout print hex values separated by 4 on a linux x86 machine. 回答1: cout << cout is equivalent to cout << cout.operator void *() . This is the idiom used before C++11 to determine if an iostream is in a failure state, and is implemented in std::ios_base ; it usually returns the address of static_cast<std::ios_base *>(

Why is the address of this volatile variable always at 1?

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问题 I wanted to inspect the address of my variable volatile int clock; cout << &clock; But it always says that x is at address 1. Am i doing something wrong?? 回答1: iostreams will cast most pointers to void * for display - but no conversion exists for volatile pointers. As such C++ falls back to the implicit cast to bool . Cast to void* explicitly if you want to print the address: std::cout << (void*)&clock; 回答2: There's an operator<< for const void* , but there's no operator<< for volatile void*

Hide user input on password prompt [duplicate]

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Possible Duplicate: Read a password from std::cin I don't work normally with the console, so my question is maybe very easy to answer or impossible to do . Is it possible to "decouple" cin and cout , so that what I type into the console doesn't appear directly in it again? I need this for letting the user typing a password and neither me nor the user normally wants his password appearing in plaintext on the screen. I tried using std::cin.tie on a stringstream , but everything I type is still mirrored in the console. From How to Hide Text : Windows #include <iostream> #include <string> #include

Why “cout” works weird for “unsigned char”?

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问题 I have the following code: cvtColor (image, image, CV_BGRA2RGB); Vec3b bottomRGB; bottomRGB=image.at<Vec3b>(821,1232); When I display bottomRGB[0] , it displays a value greater than 255. What is the reason for this? 回答1: As you have commented, the reason is that you use cout to print its content directly. Here I will try to explain to you why this will not work. cout << bottomRGB[0] << endl; Why "cout" works weird for "unsigned char" ? It will not work because here bottomRGB[0] is a unsigned

Synchronizing STD cout output multi-thread

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问题 Latelly I've been working with multi-thread coding, after a while writing I realized that if I used std::cout in different boost::threads, the output would came without a logical order, the program that I'm testing is something like: #include <boost/thread/thread.hpp> #include <iostream> int my01( void ) { std::cout << "my01" << std::endl; return 0; } /* my02, my03 and my04 are the same with different outputs*/ [...] int main( void ) { boost::thread t1(&my01); boost::thread t2(&my02); boost: