将lambda传递到功能模板中 [英] Passing a lambda into a function template
问题描述
我正在学习C ++,并且正在尝试实现一个二进制搜索功能,该功能查找谓词所持有的第一个元素.函数的第一个参数是向量,第二个参数是对给定元素的谓词求值的函数.二进制搜索功能如下:
I'm learning C++, and I'm trying to implement a binary search function that finds the first element for which a predicate holds. The function's first argument is a vector and the second argument is a function that evaluates the predicate for a given element. The binary search function looks like this:
template <typename T> int binsearch(const std::vector<T> &ts, bool (*predicate)(T)) {
...
}
如果像这样使用,它将按预期工作:
This works as expected if used like this:
bool gte(int x) {
return x >= 5;
}
int main(int argc, char** argv) {
std::vector<int> a = {1, 2, 3};
binsearch(a, gte);
return 0;
}
但是,如果我使用lambda函数作为谓词,则会出现编译器错误:
But if I use a lambda function as a predicate, I get a compiler error:
search-for-a-range.cpp:20:5: error: no matching function for call to 'binsearch'
binsearch(a, [](int e) -> bool { return e >= 5; });
^~~~~~~~~
search-for-a-range.cpp:6:27: note: candidate template ignored: could not match 'bool (*)(T)' against '(lambda at
search-for-a-range.cpp:20:18)'
template <typename T> int binsearch(const std::vector<T> &ts,
^
1 error generated.
以上错误是由
binsearch(a, [](int e) -> bool { return e >= 5; });
怎么了?为什么编译器不相信我的lambda具有正确的类型?
What's wrong? Why is the compiler not convinced that my lambda has the right type?
推荐答案
您的函数binsearch
将函数指针作为参数. lambda 和函数指针是不同的类型:lambda可以视为实现operator()
的结构的实例.
Your function binsearch
takes a function pointer as argument. A lambda and a function pointer are different types: a lambda may be considered as an instance of a struct implementing operator()
.
请注意,无状态lambda(不捕获任何变量的lambda)可以隐式转换为函数指针.由于模板替换,此处的隐式转换不起作用:
Note that stateless lambdas (lambdas that don't capture any variable) are implicitly convertible to function pointer. Here the implicit conversion doesn't work because of template substitution:
#include <iostream>
template <typename T>
void call_predicate(const T& v, void (*predicate)(T)) {
std::cout << "template" << std::endl;
predicate(v);
}
void call_predicate(const int& v, void (*predicate)(int)) {
std::cout << "overload" << std::endl;
predicate(v);
}
void foo(double v) {
std::cout << v << std::endl;
}
int main() {
// compiles and calls template function
call_predicate(42.0, foo);
// compiles and calls overload with implicit conversion
call_predicate(42, [](int v){std::cout << v << std::endl;});
// doesn't compile because template substitution fails
//call_predicate(42.0, [](double v){std::cout << v << std::endl;});
// compiles and calls template function through explicit instantiation
call_predicate<double>(42.0, [](double v){std::cout << v << std::endl;});
}
您应该使函数binsearch
更通用,例如:
You should make your function binsearch
more generic, something like:
template <typename T, typename Predicate>
T binsearch(const std::vector<T> &ts, Predicate p) {
// usage
for(auto& t : ts)
{
if(p(t)) return t;
}
// default value if p always returned false
return T{};
}
从标准算法库中汲取灵感.
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