C++ 错误：将“const ...”作为“...”的“this”参数传递会丢弃限定符
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error: passing ‘const ...'’ as ‘this’ argument of ‘...’ discards qualifiers
提问by Emy
stockListType.cpp:58: instantiated from here
stockListType.cpp:58：从这里实例化
/usr/include/c++/4.2.1/bits/stl_algo.h:91: error: passing ‘const stockType' as ‘this' argument of ‘bool stockType::operator<(const stockType&)' discards qualifiers
/usr/include/c++/4.2.1/bits/stl_algo.h:92: error: passing ‘const stockType' as ‘this' argument of ‘bool stockType::operator<(const stockType&)' discards qualifiers
/usr/include/c++/4.2.1/bits/stl_algo.h:94: error: passing ‘const stockType' as ‘this' argument of ‘bool stockType::operator<(const stockType&)' discards qualifiers
/usr/include/c++/4.2.1/bits/stl_algo.h:98: error: passing ‘const stockType' as ‘this' argument of ‘bool stockType::operator<(const stockType&)' discards qualifiers
/usr/include/c++/4.2.1/bits/stl_algo.h:100: error: passing ‘const stockType' as ‘this' argument of ‘bool stockType::operator<(const stockType&)' discards qualifiers
Above is the error I got and would like someone to explain to me what it means. I solved the error by placing a constant in front of the overloading operator. My program was a stock market application that read a file that includes a string, 5 doubles and an int. We sort out the program by the string symbols and the index gain. The book instructed me to use vectors to store each data. As you see below the overload operator compares each symbol and sorts it out using the sort member function of containers. My question is why did I have to put a constant in front of the overload operator for > and <. but not for >=, <=, ==, != overload operators.
以上是我得到的错误，希望有人向我解释它的含义。我通过在重载运算符前面放置一个常量来解决这个错误。我的程序是一个股票市场应用程序，它读取包含一个字符串、5 个双精度数和一个整数的文件。我们通过字符串符号和索引增益来整理程序。这本书指示我使用向量来存储每个数据。正如您在下面看到的，重载运算符比较每个符号并使用容器的 sort 成员函数对其进行排序。我的问题是为什么我必须在 > 和 < 的重载运算符前放置一个常量。但不适用于 >=、<=、==、!= 重载运算符。
//function was declared in stockType.h and implemented in stockType.cpp
bool operator<(const stockType& stock)//symbol is a string
{
return (symbols < stock.symbols)
}
//The function below was defined in stockListType.h and implemented in
// stockListType.cpp where I instantiated the object of stockType as a vector.
//vector<stockType> list; was defined in stockListType.h file
void insert(const& stockType item)
{
list.push_back(item);
}
void stockListType::sortStockSymbols()
{
sort(list.begin(), list.end());
}
回答by billz
The error message tells you that you that you are casting of constfrom your object in operator<function. You should add constto all member functions that don't modify member.
错误消息告诉您，您正在const从operator<函数中的对象进行转换。您应该添加const到所有不修改成员的成员函数。
bool operator<(const stockType& stock) const
// ^^^^^
{
return (symbols < stock.symbols)
}
The reason why compiler complains about operator<is because std::sortuses operator<to compare the elements.
编译器抱怨的原因operator<是因为std::sort用于operator<比较元素。
Also you have another syntax error in insertfunction.
此外，您还有另一个语法错误insert。
Update:
更新：
void insert(const& stockType item);
to:
到：
void insert(const stockType& item);
// ^^
