关于创建Name_class类、vector存两类数据及构造函数初始化列表的咨询

Hey there! Let's break down your questions step by step, nice and clear.

That line Name_value(std::string n, int v) :name(n), value(v) { } is what's called a constructor initializer list in C++. It's the most efficient (and sometimes mandatory) way to initialize class member variables. Let's unpack it:

• Name_value(std::string n, int v): This is the constructor signature—it takes a std::string and an int as inputs to create a Name_value object.
• :name(n), value(v): This is the initializer list. Right after the colon, we list each member variable we want to initialize, paired with the value we want to give it (separated by commas):
  • name(n): This directly initializes the class member name using the input parameter n. Think of it as setting the value of name the moment it's created, rather than creating it empty then assigning a value later.
  • value(v): Same logic here—we initialize the int member value with the input v as soon as it exists.
• { }: The empty function body means all initialization work is already done in the initializer list; no extra assignment steps are needed here.

To put it in perspective, here's the less efficient alternative using in-body assignment:

Name_value(std::string n, int v) {
    name = n;  // name was already default-initialized, now we overwrite it
    value = v; // same for value
}

The initializer list skips that default initialization step, making it faster. Plus, for certain members like const variables or references, you can't use in-body assignment—you have to use the initializer list.

Your existing Name_value class already does the heavy lifting of wrapping two data types (std::string and int) into one object. Let's rename it to Name_class and show how to use a std::vector to store multiple instances of it:

#include "std_library_facilities.h"
void error(std::string b) { throw std::runtime_error(b); }

class Name_class {
public:
    std::string name;
    int value;
    // We'll keep using the initializer list for efficiency
    Name_class(std::string n, int v) : name(n), value(v) { }
};

int main() {
    // Create a vector that holds Name_class objects
    std::vector<Name_class> name_collection;

    // Add objects to the vector (emplace_back constructs them directly in the vector)
    name_collection.emplace_back("Alice", 28);
    name_collection.emplace_back("Bob", 32);
    name_collection.emplace_back("Charlie", 25);

    // Loop through and print the stored data
    for (const auto& item : name_collection) {
        std::cout << "Name: " << item.name << ", Value: " << item.value << "\n";
    }

    return 0;
}

By storing Name_class objects in the vector, you're effectively storing pairs of string and int values in a single container. Each Name_class instance bundles the two data types together, so the vector only needs to handle one type of object.

内容的提问来源于stack exchange，提问作者x blade