const filterMeta:StateType['filterMeta'] = {

In C++, using `const` reference when passing arguments to functions can improve efficiency[^1]. Here's an example of a function `isprint` that takes a `std::string` by const reference and checks if all characters are printable:

// 示例函数，通过const引用检查字符串中的字符是否可打印
bool isprint(std::string const &s) {
    return all_of(begin(s), end(s), [](char c){ return isprint(c); });
}

Now, regarding the snippet about class `A` and function `B`, if we have a member variable `filterMeta` in some `StateType` structure, and you want to pass it as a constant reference to a function, you might do something like this:

// 假设有一个名为FilterMeta的状态类型，其中包含filterMeta成员
struct StateType {
    FilterMeta filterMeta;
};

void processFilter(const StateType &state) {
    // 在这里处理const引用filterMeta，不需要复制整个结构
    // ...
    B(state.filterMeta);
}

// 调用函数，传递const引用
void A() {
    StateType state;
    // 初始化filterMeta...
    processFilter(state);
}

Here, `processFilter` takes a `const` reference to `StateType`, allowing it to modify `filterMeta` without creating a new copy.