有条件地调用cv :: VideoCapture构造函数

Question

I want to call the cv::VideoCapture constructor conditionally. If argc == 2 , then the user has specified a video file and I want to load that. Otherwise use the default camera.

#include <opencv2/opencv.hpp>
#include <iostream>

using namespace cv;

int main(int argc, char* argv[])
{
    VideoCapture cap(argc == 2 ? argv[1] : 0);

    return 0;
}

I compile and run it

$ g++ -g -Wall -o main main.cpp `pkg-config opencv --cflags --libs`
$ ./main

But sadly it doesn't seem to work and I get

OpenCV: Couldn't read movie file ""

The ternary operator returns a string in one case and an integer in another. I reckon this won't work because of some idiosyncrasy of C++ constructors I'm not aware of.

What's the cleanest way to do what I want? Can I do it without using new so that I don't have to free memory myself at the end of the program?

Answer 1

You're over-complicating this. Use open :

VideoCapture cap;
if(argc == 2) {
    cap.open(argv[1]); // calls: open(const String& filename)
} else {
    cap.open(0);       // calls: open(int index) 
}

---

In the ternary operator the two operands must be of the same type . See here for details. But yours have type int and char* , so, even of the code may compile, it can result in runtime errors.

Answer 2

Expression A ? B : C A ? B : C has a return type of B and C must be implicitly convertible to B . Some compilers will not compile (best behavior), some will. In your case, B = char * and C = int . I believe 0 is interpreted as pointer to address 0 or NULL whose byte equal '\\0' ie null-termination character that marks the end of a C-style string hence the empty string.

If you insist on a one-liner, then:

std::unique_ptr<VideoCapture> capture = (argc == 2 ? std::make_unique<VideoCapture>(argv[1]) : std::make_unique<VideoCapture>(0));