C ++ lambda模糊调用 [英] C++ lambda ambiguous call
本文介绍了C ++ lambda模糊调用的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我试图每个都有两个Pixel函数.一个返回图像,另一个不返回任何图像.即使指定了lambda的返回类型,我也仍然对eachPixel的调用不明确".
I am trying to have two eachPixel functions. One returns an image, and the other returns nothing. I'm getting "call to eachPixel is ambiguous" even though I am specifying the return type of the lambda.
如何解决歧义?
// this one does not return an image
void eachPixel(const QImage &image, const std::function <void (uint)>& pixelFunc, const QRect &bounds=QRect()) {
...
for (int y=region.y(); y<=region.bottom(); y++) {
for (int x=region.x(); x<=region.right(); x++) {
pixelFunc(image.pixel(x,y));
}
}
}
// This one returns an image
QImage eachPixel(const QImage &image, const std::function <uint (uint)>& pixelFunc, const QRect &bounds=QRect()) {
...
QImage out(image.size(), image.format());
for (int y=region.y(); y<=region.bottom(); y++) {
for (int x=region.x(); x<=region.right(); x++) {
out.setPixel(x,y, pixelFunc(image.pixel(x,y)));
}
}
return out;
}
void test_pixelFunc() {
QImage image(300,200, QImage::Format_ARGB32);
image.fill(Qt::blue);
QImage out = eachPixel(image, [] (uint p) -> uint { //uint specified!!
return qRgb(qRed(p), qBlue(p), qGreen(p)); // swap green and blue channels
}, QRect (0,0, 300, 200));
out.save("test_pixelfunc.png");
int accumulator=0;
eachPixel(image, [&accumulator](uint p) -> void { // void specified!
accumulator++;
}, QRect (0,0, 300, 200));
qDebug() << "accumulator" << accumulator;
};
推荐答案
您可以在传入的函数的返回类型上使用模板和SFINAE.
You can use templates and SFINAE on the return type of the function you pass in.
#include <iostream>
#include <functional>
template <typename T, std::enable_if_t<std::is_same_v<void, decltype(std::declval<T>()(1))>, int> = 0>
void foo (T f) {
// std::function<void(int)> func = f;
// if you really need a std::function
f(1);
}
template <typename T, std::enable_if_t<std::is_same_v<int, decltype(std::declval<T>()(1))>, int> = 0>
int foo (T f) {
return f(1);
}
int main() {
foo([](int x) { std::cout << "void " << x << '\n'; });
foo([](int x) { std::cout << "int\n"; return x; });
}
对于c ++ 11,您可以直接使用std::is_same
和std::enable_if
.
For c++11 you can use std::is_same
and std::enable_if
directly.
template <typename T, typename std::enable_if<std::is_same<void, decltype(std::declval<T>()(1))>::value, int>::type = 0>
void foo (T f) {
f(1);
}
template <typename T, typename std::enable_if<std::is_same<int, decltype(std::declval<T>()(1))>::value, int>::type = 0>
int foo (T f) {
return f(1);
}
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