在c ++的构造函数中，const字符串和名称以及字符串名称之间有所不同

Question

Person(const string &name)
{
    mName=name;
}


Person(string name)
{
    mName=name;
}

mName is a private member variable

So I made a class called Person. I'm wondering whats the difference between const string &name and string name. I have tried just putting string &name but it gave me an error message.

I know & is a reference so almost like an address? I'm guessing the const is needed because the reference is a constant? Also, why doesn't just string &name work?

This is the error message that I got

candidate constructor (the implicit copy constructor) not viable: no known conversion from 'const char [4]' to 'const Person' for 1st argument

candidate constructor not viable: no known conversion from 'const char [4]' to 'string &' (aka 'basic_string, allocator > &') for 1st argument Person(string &name)

Answer 1

Option #1 - pass the argument by-value, with Person(string name) :

1. string constructor is called, and a temporary string instance is created on the stack.

2. Person constructor is called.

3. string destructor is called with the temporary instance.

Option #2 - pass the argument by-reference, with Person(const string& name) :

1. A reference to the string instance (its 4-byte or 8-byte address) is placed on the stack.

2. Person constructor is called.

As you can see, option #2 is generally more efficient.

The compilation error is probably the result of something like Person x("abc") :

The compiler first searches for a Person constructor which takes the explicit type of the argument that you are passing. In this case, it is Person(const char[]) , which you have not defined.

Then, the compiler searches for any other Person constructor which takes a "convertable" type. Since operator const string&(const char[]) exists, the compiler can convert the argument "abc" from const char[] to const string& . So if you define Person(const string& name) , then your code is successfully compiled.

Finally, the compiler explains for each Person constructor that you've defined, why it is not a suitable candidate for your call. In this case, it tells you that there is no operator string&(const char[]) , which allows it to convert the argument "abc" from const char[] to string& before calling your Person(string&) constructor.

Answer 2

&name is a reference which means it is just another name for the string you pass in as an argument. This allows you to pass in the object itself, not just its value. You have to use const if you want to pass in a const. A const cannot be modified.

Answer 3

&name is a reference to name, so the actual object is passed as a parameter, not a copy. When you put const string &name, name is still being passed as a reference, but the const makes it so you cannot alter the actual object, just use it.

Answer 4

const string &name与string name之间的区别在于， const string &name是对字符串的常量引用，此处在将其传递给函数时不会创建另一个副本，也不能在函数内部更改它，因为它是常量。