c++ - What does argc mean? -

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• opencv argc , argv confusion 1 answer

i can't understand, whats function of code in opencv loading image. whats function of if(argc !=2)? can tell me it.

 if( argc != 2)     {      cout <<" usage: display_image imagetoloadanddisplay" << endl;      return -1;     } 

full code:

1 #include <opencv2/core/core.hpp> 2 #include <opencv2/highgui/highgui.hpp> 3 #include <iostream> 4 5 using namespace cv; 6 using namespace std; 7 8 int main( int argc, char** argv ) 9 { 10 if( argc != 2) 11 { 12 cout <<" usage: display_image imagetoloadanddisplay" << endl; 13 return -1; 14 } 15 16 mat image; 17 image = imread(argv[1], cv_load_image_color); // read file 18 19 if(! image.data ) // check invalid input 20 { 21 cout << "could not open or find image" << std::endl ; 22 return -1; 23 } 24 25 namedwindow( "display window", cv_window_autosize );// create window display. 26 imshow( "display window", image ); // show our image inside it. 27 28 waitkey(0); // wait keystroke in window 29 return 0; 30 } 

this should covered in tutorial c++ (or c, objc, or related languages), such the gnu tutorial.

the main function of c++ program has 2 parameters, convention named argc , argv, give command-line arguments used launch program.

argc count of arguments, , argv array of strings.

the program first argument, argv[0], argc @ least 1.

so, argc 2 when program run 1 command-line argument. if it's run no arguments, or more one, argc != 2 true, usage message " usage: display_image imagetoloadanddisplay" printed, telling user how run properly.

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for example, if this:

$ display_image firstarg "second arg" 

the values be:

argc: 3 argv[0]: "display_image" argv[1]: "firstarg" argv[2]: "second arg" 

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it may worth pointing out code weird in many ways. space @ beginning of usage message weird. "usage" in lowercase. typically put actual program name (argv[0]) in string rather hardcoded canonical name. usage messages go cerr, not cout. , convention return positive number errors user's fault, typically 2 invalid arguments, not -1. can find better examples of argc/argv handling in source code tool made used on unix command line (although of them more complicated, using libraries getopt parse out options file arguments, etc.).