将 std::tolower 与 std::transform 一起使用

Question

我想了解为什么完全限定std::结果会导致下面的编译错误，使用std::transform.

#include <algorithm>
int main() {
    std::string str;
    std::transform(str.begin(), str.end(), str.begin(), std::tolower); // does not compile

    std::transform(str.begin(), str.end(), str.begin(), ::tolower); // OK
}

无法推导一元运算符的错误。

source>: In function 'int main()':
<source>:4:19: error: no matching function for call to 'transform(std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, <unresolved overloaded function type>)'
    4 |     std::transform(str.begin(), str.end(), str.begin(), std::tolower);
      |     ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/algorithm:62,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:4285:5: note: candidate: 'template<class _IIter, class _OIter, class _UnaryOperation> constexpr _OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation)'
 4285 |     transform(_InputIterator __first, _InputIterator __last,
      |     ^~~~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:4285:5: note:   template argument deduction/substitution failed:
<source>:4:19: note:   couldn't deduce template parameter '_UnaryOperation'
    4 |     std::transform(str.begin(), str.end(), str.begin(), std::tolower);
      |     ~~~~~~~~~~~~~~^~~~~~~

Answer 1

您需要指定您想要 std 命名空间中的哪一个函数：tolower

std::transform(str.begin(), str.end(), str.begin(), static_cast<int(*)(int)>(std::tolower));

请注意，您还缺少string并cctype包含。