这是c ++模板param deduction不正确吗？ [英] Is this c++ template param deduction incorrect?

问题描述

#include <iostream>
#include <regex>
int main(void)
{
    std::cmatch cm;
    std::regex_match("subject", cm, std::regex("(sub)(.*)"));
    //std::for_each(cm.begin(), cm.end(), [](const std::sub_match<const char *> &s){   <---- Working statement

    std::for_each(cm.begin(), cm.end(), [](const std::cmatch &s){ /*<--- Non-working statement*/
        std::cout << "match:" << s.str() <<std::endl;
    });
    return 0;
}

错误如下：

 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:853:9: error: no matching function for call to object of type '(lambda at main.cpp:73:41)'
 __f(*__first);
 ^~~
 main.cpp:73:10: note: in instantiation of function template specialization 'std::__1::for_each<std::__1::__wrap_iter<const std::__1::sub_match<const char *> *>, (lambda at main.cpp:73:41)>' requested here
 std::for_each(cm.begin(), cm.end(), [](const std::match_results<const char*> &s){
 ^
 main.cpp:73:41: note: candidate function not viable: no known conversion from 'const std::__1::sub_match<const char *>' to 'const std::match_results<const char *>' for 1st argument
 std::for_each(cm.begin(), cm.end(), [](const std::match_results<const char*> &s){
 ^
 maintool.cpp:73:41: note: conversion candidate of type 'void (*)(const std::match_results<const char *> &)'
 1 error generated.

$ b $ p

在非工作示例中，为什么模板被推导为 std :: __ 1 :: for_each< std :: __ 1 :: __ wrap_iter< const std :: __ 1 :: sub_match< const char *> *> ？

我期望param将被推导为 std :::cmatch
你能解释一下param deduction是如何工作的？

I was expecting param will be deduced to be std:::cmatch Can you please explain how param deduction is working here?

推荐答案

std :: cmatch 是 std :: match_results< char const *> ;您想要 std :: sub_match< char const *> std :: csub_match 。

std::for_each(cm.begin(), cm.end(), [](const std::csub_match &s) { ... }

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