一个与map下标操作有关的编译错误（花了5分钟才找到原因）

来源：互联网发布：python数据分析视频编辑：程序博客网时间：2024/05/05 09:53

来，看程序：

#include <iostream>#include <string>#include <map>using namespace std;class A{public:map<string, string> m_map;};void test(const A &a){string s = a.m_map["hello"];cout << s << endl;}int main(){A a;a.m_map["hello"] = "world";test(a);return 0;}

g++编译错误如下：

main.cpp: In function ‘void test(const A&)’:
main.cpp:14: error: passing ‘const std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >’ as ‘this’ argument of ‘_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _Tp = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _Compare = std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, _Alloc = std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >]’ discards qualifiers

原因：在test中， a的值不能变，而a.m_map["hello"]是一个可读写操作，所以有问题。还是用const_iterator吧，先查找"hello", 再访问it->second

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