C ++运算符重载，我自己的字符串类

Question

I'm trying to make my own string class in c++11 but I have some problems. comparing my class to the std::string class, I can't figute out how to use the

std::string.at(int) = 'a'; method/overloading.

I've created the at(int) method in my own class:

int at(int index)
{
    if(index <0 || index > size-1) {throw std::out_of_range("Error, index out of range");}
    return data[index];
}

and it workd well if I only use:

MyString.at(2);

In the main file:

MyString = "Hello world!"; //Works fine!
MyString.at(2)='a'; //Gives, Error: expressino must be a modifiable Ivalue.

Any help with this would be great, Thanks!

Answer 1

At least one of your at() member functions needs to return a non-const reference to char . Like this:

char &at(std::size_t idx)
{
    return data[idx];
}

It would be beneficial to define a const version of the function too:

const char &at(std::size_t idx) const
{
    return data[idx];
}

Also note the use of std::size_t (which is an unsigned type guaranteed to be large enough to represent any size). This way you improve portability and you don't have to check for negative indices.

Answer 2

You are returning an integer and not a reference to the character, you probably want:

char& at(int index)

Of course, you need to return the correct character type, but in any case you need to return a reference in order for the caller to be able to assign to it.