Member Initialization Lists in C
In C, the member initialization list is used to initialize member variables in the constructor, especially for const members, reference members, class members without default constructors, and performance optimization. Its syntax begins with a colon and is followed by a comma-separated initialization item. Reasons for using member initialization list include: 1. The const member variable must be assigned value at initialization; 2. The reference member must be initialized; 3. Class type members without default constructors need to explicitly call the constructor; 4. Improve the construction efficiency of class type members. In addition, the initialization order is determined by the order of members declared in the class, not the order in the initialization list, so be careful to avoid relying on uninitialized members. Common application scenarios include initialization constants, references, complex objects and members that need to be passed parameter structures.
In C, the Member Initialization List is a very critical part of the constructor. Many people tend to ignore it when they first start writing classes, or just use assignments to process member variables. But in fact, in some cases, initialization list must be used, otherwise the code may be error-free or inefficient.
Let’s talk about when to use it, why, and how to use it.
What is a member initialization list?
The member initialization list appears in the definition of the constructor, immediately following the parameter list, starting with a colon, followed by a series of initialization expressions of member variables, each separated by a comma, and the last one is followed by the function body.
for example:
class MyClass {
int a;
std::string name;
public:
MyClass(int val, const std::string& str) : a(val), name(str) {
// Constructor body}
}; Here : a(val), name(str) is the member initialization list.
It is different from writing a = val; name = str; in the body of the constructor, especially on some types, this difference will affect whether the program can be compiled or run correctly.
Why can't we always use constructor body to assign values?
There are several main reasons:
const member variable : Once declared as const, the value must be specified during initialization and cannot be assigned in the constructor body.
class A { const int x; public: A(int val) { x = val; } // Error! const variable cannot be assigned };The correct way to do it is to use the initialization list:
A(int val) : x(val) {}Reference member variables : References must also be initialized and cannot be declared before assignment.
Class type members without default constructor : If your class contains a child object without default constructor, then if you do not explicitly call its constructor, you will report an error.
for example:
class B { public: B(int x) { /* ... */ } }; class A { B b; public: A() { } // Error! B There is no default constructor, it cannot be constructed automatically};Must be written like this:
A(int val) : b(val) { }Performance optimization : For class type member variables, using the initialization list can avoid the process of calling the default constructor first and then assigning values, and directly constructing it in one go.
The execution order of the member initialization list
The initialization order of member variables is not in the order in the initialization list , but in the order in which they are declared in the class.
For example:
class MyClass {
int a;
int b;
public:
MyClass() : b(20), a(b) {}
}; Although you write b(20) in the list first and then a(b) , because a is declared in the class before b , a will be initialized first. At this time, b has not been initialized, and the result is that the value of a is undefined.
Pay special attention to this point, otherwise it is easy to write bug-related code.
Common usage scenarios in actual development
- Initialize a constant member (const)
- Initialize the reference member
- Initialize object members without default constructor
- Improve performance, especially for complex objects (such as containers, custom types)
For example, if you have a class that needs to pass in configuration information:
class Config {
public:
Config(const std::string& filename) { /* Read configuration from file*/ }
};
class App {
Config config_;
public:
App(const std::string& file) : config_(file) {}
}; The initialization list cannot be omitted here, because Config does not have a default constructor.
Basically that's it. The member initialization list looks simple, but if you don't pay attention to it in actual development, it is easy to get stuck. Especially when it comes to const, references and complex objects, be sure to remember to use them well.
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