从std :: basic_string撰写cv :: Mat图片<char>

Question

I am trying to compose cv::Mat image from std::basic_string<char> but the composed image is empty. Below is the code snippet:

std::basic_string<char> color_pixels = color_frame.data();
cv::Mat image(cv::Size(color_frame.width(), color_frame.height()),
              CV_8UC4,
              &color_pixels,
              cv::Mat::AUTO_STEP);

Before proceeding further, I wanted to verify the data. Hence I checked width , height etc and found correct.

std::cout << "width:" << color_frame.width() <<
          ", height:" << color_frame.height() << 
          ", length:" <<  color_pixels.length() << std::endl;
width:1920, height:1080, length:8294400 // 1920 * 1080 * 4 = 8294400

I found a similar question here and tried to implement in my code as follows:

cv::Mat image(cv::Size(color_frame.width(), color_frame.height()),
              CV_8UC4,
              color_pixels.data(),
              cv::Mat::AUTO_STEP);

Unfortunately, it is throwing following error:

error: invalid conversion from ‘const void*’ to ‘void*’

How to compose cv::Mat Image from std::basic_string? Any workaround, please?

Answer 1

color_pixels.data() is a const value.

http://en.cppreference.com/w/cpp/string/basic_string/data

Answer 2

In C++, void* can be used to represent a pointer to some unspecified, valid memory address (internally, this will be either passed to some low-level API, like C memcpy, or casted to some meaningful type according to the given interface contract). IMHO, this is seldom if ever necessary in the modern C++ type system (and function dealing with memory can use std::byte now), but C++ libraries using it sadly still exist.

Anyway, coming back to your question, you can implicitly convert any pointer to void*, but you cannot implictly cast-away constness. You need const_cast for that:

cv::Mat image(cv::Size(color_frame.width(), color_frame.height()),
              CV_8UC4,
              const_cast<char*>(color_pixels.data()),
              cv::Mat::AUTO_STEP);

---

A few caveats:

• in >=C++11, if color_pixels were originally const, any attempt to modify color_pixels data via image will result in undefined behaviour .

• in C++<=03, data() were not required to point to the original string characters, so any modification attempt would be UB in that case.

• invoking color_pixels modifiers invalidating its content will make invoking most image methods UB as well.