different between const string &name and string name in constructor for c++

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)

Post the error message that it gives you when trying to use a non-const reference. — Yuushi
the error message is related to the data type of mName. What data type have you used, Can you post the full code? — vathsa

barak manos barak manos · Accepted Answer · 2014-02-07T06:46:39

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

string constructor is called, and a temporary string instance is created on the stack.
Person constructor is called.
string destructor is called with the temporary instance.

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

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

different between const string &name and string name in constructor for c++

4 Answers