如何将项目放入const向量？

Question

I've been trying a bunch of different ways, but I can't figure it out. The declaration that I was given is:

const std::vector<std::string>&,

I've been trying to do this:

gradeReported.push_back(firstEntry);

I keep getting an error. I am pretty sure it has something to do with the const nature of the vector. Help would be appreciated!

Answer 1

You don't. If it's const , it cannot be modified. You need to make a copy or take it by non-const reference.

Answer 2

You can't put items into a const vector, the vectors state is the items it holds, and adding items to the vector modifies that state.

If you want to append to a vector you must take in a non const ref.

void f(std::vector<int>& v)
{ 
    v.push_back(4);
}

Answer 3

If you have a const vector it means you can only read the elements from that vector.
The same rules apply to a vector as to any other type when you use the const qualifier.

To be able to modify it, You will either have to make a copy of the original vector or take a non const reference.

Answer 4

The whole point of a const vector is that you cannot modify it. Appending an element to it would be a modification and thus a violation of its constness. That's why you can't do it. It's by design.

Answer 5

You're not supposed to be able to change it, that's kind of the point of the const. It's mostly used for passing in parameters to functions/methods and having an assurance that it won't be modified.

If you really need to change it, there are two options:

1. Make a copy of the vector and modify that instead.
2. Remove the const via a const_cast.

Making a copy

const std::vector<std::string>& foo;
std::vector<std::string> bar(foo);

Removing const via const_cast

I would strongly recommend against using this, but it is possibly an option.

Assuming that the string vector is being given const protection before being passed as a parameter to a method or a function, and that the underlying type is not actually const , then the following can be done:

typedef std::vector<std::string> string_vector;
void f(const string_vector& foo) {
    const string_vector& foo;
    const_cast<string_vector>(foo).push(firstEntry);
}

int main() {
    string_vector foo();
    f(foo);
}

Answer 6

你必须在你的声明中删除const，或者如果不可能使用它

const_cast<std::vector<std::string>& >(gradeReported).push_back(firstEntry);