C++ 使用 function 初始化结构

Question

I'm dealing with the following situation:

Struct Data;
typedef std::shared_ptr<Data> DataPtr;
typedef std::unordered_map<int, DataPtr> DataPtrMap;

DataPtrMap data;
std::string file_path;

I need to create an empty DataPtrMap and initialize it from file with a function that takes the file path as an input. Essentially, this function reads the file line-by-line, creates a new struct , fills it with data and inserts it in the unordered_map . The Data structure contains just some integers and a couple of small matrices, and there can be up to a few thousands of them in the unordered_map . I'm wondering whether it's more efficient to pass a pointer to the (initially empty) data as input to the function as:

void loadData(DataPtrMap*, const std::string&);
loadData(&data, file_path);

or if it's better to directly return it from the function as:

DataPtrMap loadData(const std::string&);
data = loadData(file_path);

My intuition is that the second solution is much cleaner in terms of readability and return value optimization should avoid making a useless copy of the data when returning from the function. However, I would like to have the opinion of more expert coders.

Thank you in advance for the help.

Answer 1

To avoid the use of raw pointer in your case you can use references in your option1:

void loadData(DataPtrMap&, const std::string&);
loadData(data, file_path);

With references the first option is clean and needs no more optimization to avoid unnecessary copy.

I like the syntax of option2 but your need to take care of optimizing copy. But like jfh said, optimization is the last step when coding, do not bother with that at the beginning