std :: map存取操作符已弃用?没有operator []匹配这些操作数 [英] std::map access operator deprecated? no operator [] matches these operands
问题描述
根据 http://www.cplusplus.com/reference/map/map/ ,我可以使用 m [k]
或 m.at(k)
地图中的键 k
m
。但是,当我尝试做
According to http://www.cplusplus.com/reference/map/map/, I can use either m[k]
or m.at(k)
to access the value of a key k
in a map m
. However, when I try to do
derivMap[fx]
在我的代码中,其中derivMap是 std :: map
in my code, where derivMap is an element of type std::map<std::string,std::string>
Visual Studio 2013 gives me the warning
无操作符[]匹配这些操作数
no operator [] matches these operands
但是,当我将代码更改为
However, when I change my code to
derivMap.at(fx)
我没有错误。您对此问题有任何见解吗?
I get no error. Do you have any insight into this issue?
推荐答案
map :: operator []
不会弃用。
map::operator[]
is not deprecated.
我猜想你在 derivMap
是 const
的上下文中尝试调用运算符。 map :: operator []
没有 const
重载,因为它可以通过插入元素来修改地图一个匹配的键不存在。 map :: at()
另一方面,有一个 const
重载,因为它被设计为当元素。
I will guess that you are attempting to call the operator in a context where derivMap
is const
. map::operator[]
does not have a const
overload, because it can modify the map by inserting an element when one matching the key isn't present. map::at()
on the other hand, does have a const
overload because it is designed to throw when an element is not found.
void foo(std::map<int, int>& m)
{
int n = m[42]; // OK
}
void bar(const std::map<int, int>& m)
{
int n = m[42]; // ERROR
}
这篇关于std :: map存取操作符已弃用?没有operator []匹配这些操作数的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!