当我使用[=]捕获(使用Lambda类)时，为什么变量变成'const'? [英] Why does a variable become 'const' when I use a [=] capture (using Lambda class)?

问题描述

前提#1 :我已经解决了该错误，但是我不十分了解编译器错误的原因.

Premise #1: I have already solved the error, but I didn't deeply understand the cause of the compiler error.

前提2 :该程序的目标是通过多线程进程将一个映像复制到另一个映像中.也许存在更好的方法，但这不是问题的重点(请参阅前提1).

Premise #2: The goal of this program is to copy a image into another image by a multithreaded process. Maybe a better way exists, but this is not the focus topic of the question (see premise #1).

我使用OpenCV 3.1库编写了一个简单程序，可以将一个图像复制到另一个图像中.它利用了使用更多线程的CPU所有内核.

I wrote a simple program using OpenCV 3.1 library to copy a image into another image. It takes advantage of all cores of the CPU using more threads.

代码是:

#include <opencv2/opencv.hpp>
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <thread>

using namespace cv;
using namespace std;

#define IMG_PATH "..\\img\\50a.png"


void copy_image(const Mat& input, Mat& output, int row_offset, int skip_row)
{
    cout << row_offset;
    Size size = input.size();
    for (int r = row_offset; r < size.height; r += skip_row)
    {
        for (int c = 0; c < size.width; c++)
        {
            output.at<Vec3b>(r, c) = input.at<Vec3b>(r, c);
        }
    }
}

int main()
{
    Mat input_img = imread(IMG_PATH);
    Mat output_img = Mat(input_img.size(), input_img.type()); output_img.setTo(255);

    vector<thread> threads;
    int thread_number = thread::hardware_concurrency();

    for (int i = 0; i < thread_number; i++)
    {
        cout << i;
        auto var = [=]() 
        {
            return copy_image(input_img, output_img, i, thread_number);
        };

        threads.push_back(thread(var));
    }

    for (auto& thread : threads) 
        thread.join();

    imshow("output_img", output_img);
    imwrite("result.png", output_img);
    waitKey(0);
}

编译器向我返回此错误

错误C2664'无效的copy_image(const cv :: Mat&，cv :: Mat&，int，int)': 无法将参数2从'const cv :: Mat'转换为'cv :: Mat&'

Error C2664 'void copy_image(const cv::Mat &,cv::Mat &,int,int)': cannot convert argument 2 from 'const cv::Mat' to 'cv::Mat &'

它使用以下代码行:

return copy_image(input_img, output_img, i, thread_number);

我解决了替换此行的错误

I solved this error replacing this line

auto var = [=]()

与此

auto var = [=, &input_img, &output_img]() 

但实际上我不十分了解为什么收到该错误.

but actually I don't deeply understand why I received that error.

推荐答案

如果您在lambda中按值进行捕获，则将获得一个成员"，该成员将被存储.由于默认的operator()是const函数，因此无法对其进行修改.

If you do a capture by value in a lambda, you will get a 'member' which gets stored. As the default operator() is a const function, you cannot modify them.

可以将Lambda定义为[]() mutable {}，以允许您修改局部变量.

Lambdas can be defined as []() mutable {} to allow you to modify the local variables.

通过按引用捕获值，您的行为就像指向非const对象的const指针一样，因此如果没有可变对象，则可以适应这些对象. (除非它们已经在const中了)

By capturing the value by reference, you have something which behaves like a const pointer to a non-const object, so without the mutable, you can adapt those objects. (Unless they already where const)

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