[英]No viable overloaded operator[] for type
I have defined a type: 我已经定义了一个类型:
typedef unordered_map<string, list<string>> Graph;
I have a key: 我有一把钥匙:
string v = "A12";
When I try to access the list using the key: 当我尝试使用密钥访问列表时:
for (auto w = g[v].begin(); w != g[v].end(); w++)
{
...
}
g is type Graph
, I get the error: g是
Graph
类型,我得到错误:
No viable overloaded operator[] for type 'const Graph'
How can I fix this issue? 我该如何解决这个问题?
The problem is that g
is a const graph&
. 问题是
g
是一个const graph&
。 The indexing operator []
may need to create a new element for the map thus mutating it. 索引operator
[]
可能需要为map创建一个新元素,从而使其发生变异。 That's why you cannot use it on a const graph&
(the problem is const
). 这就是为什么你不能在
const graph&
上使用它(问题是const
)。
You can use g.at(key)
instead that throws an exception if the key is not present. 如果密钥不存在,您可以使用
g.at(key)
代替抛出异常。
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