opencv using C++
using namespace std
what does this actually mean and why do we use it?
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std points at the standard c++ namespace on your system containing all basic functionality for c++ functions. if you write it then you are allowed to just use function names and basic c++ types directly.
For example: int c = builtinFunction1(parameter2);
However if you would not use it, you would have to define the same command as
std::int c = std::builtinFunction1(parameter2);
Reason for the existance of namespaces is very simple. C++ does not deny you to redefine existing types or redefine existing functionality. This means that if I would define a namespace TEST, then I could add built-in C++ commands that act totally different as I define it.
The following command would do complete different things if I wanted it to and could be used to divert between my int type and the standard int type.
std::int c = std::builtinFunction1(parameter2); TEST::int c = TEST::builtinFunction1(parameter2);
It would be actually two complete different variables of another type. This is why most OpenCV programs also have the following command: using
namespace cv;
This in turn allows you to use commands like
Mat test = new Mat();
If the command would not have been placed, then the compiler would not recognize the type and would ask you to actually write the following to have working code:
cv::Mat test = new cv::Mat();
Hope this makes sense!
maybe it's easier with an example:
// functions are kept in a separate namespace,
// so they don't involuntarily mix or shadow others:
namespace chai
{
void foo(int);
in bar(float z);
}
// ... other code:
foo(13); // error, foo not found ( missing namespace qualifier )
chai::foo(13); // ok, correct namespace
using chai::foo; // importing only a single symbol
foo(13); // ok found.
bar(13.2); // error, only foo was imported.
using namespace chai; // importing anything from namespace chai
foo(13); // ok found.
bar(13.2); // ok found.
Asked: 2013-02-27 09:23:39 -0600
Seen: 316 times
Last updated: Feb 27 '13