C++ 向量用值初始化

Question

As a C++ beginner, I know that we can initialize a vector of vectors using:

vector<vector<int>> vec(10, vector<int>(15, 0));

which is quite efficient, but when I want to do the same thing with a struct I wrote, The code won't compile:

vector<TreeNode> vec(10, TreeNode("abc"));

the struct looks like this:

struct TreeNode {
    std::string val;
    TreeNode* left;
    TreeNode* right;
    explicit TreeNode(std::string& val) {
        this->val = val;
        left = nullptr, right = nullptr;
    }

};

I am wondering what's the problem here

I used Clion as my IDE, IDE was reporting this error: No matching conversion for functional-style cast from 'const char [4]' to 'TreeNode'

And compiler was reporting this error:

Answer 1

A string literal like "abc" isn't a std::string . But that's okay! A std::string , such as the one that's a parameter in your TreeNode constructor, can be constructed from one automatically.

However, that string will be a temporary , and temporaries don't bind to non- const references (except with a Visual Studio extension).

That's okay too: you don't need them to; your constructor can take a const std::string& instead and everything will be fine.

Other solutions include taking a std::string by value, which is actually a good idea as then you can std::move from it inside the constructor, and now your constructor is efficient whether you're passing in a temporary or moving in some other named string. But that's a story for another day.

As an aside, if this project is for production, and you are likely to have a lot more vectors than shown in your example, I strongly advise you don't nest them like this. Each element has its own memory management and indirection, which is very wasteful. Your data are square (the inner dimension always has the same size), so all you actually need is a single vector of 10*15 int s. Then those int s will all live in one nice big array, which is cache-friendly and only requires one allocation and one deallocation, Instead of, like 165. of each. Now you'll have to wrap your accesses in some maths (like index = y*width + x ) but that's trivial.

Answer 2

The problem is not in the initialization of the std::vector (which is correct), but in the initialization of the TreeNode structure.

This altered code works:

std::string initVal = "abc";
std::vector<TreeNode> vec(10, TreeNode(initVal));