比较c ++字符串

Question

class Song {

public:
    const string getAutherName();
}

void mtm::RadioManager::addSong(const Song& song,const Song& song1) {

    if (song.getAutherName() == song1.getAutherName())

}

I get this error:

Invalid arguments ' Candidates are: std::basic_string<char,std::char_traits<char>,std::allocator<char>> getAutherName() ' - passing 'const mtm::Song' as 'this' argument of 'std::string mtm::Song::getAutherName()' discards qualifiers [- fpermissive]

Why is it using basic_string and not string ! how to fix this?

Answer 1

Your getAutherName() function is not const , so it cannot be called through a const Song& . Change the function declaration like this:

class Song {

public:
    const string getAutherName() const;
}

Answer 2

You are calling getAutherName() on const Songs so you need to make that method const :

const string getAutherName() const;

It isn't clear why you return a const string . Either return a string , or return a const reference to one:

const string& getAutherName() const;
string getAutherName() const;

Answer 3

std::string is a typedef for basic_string<char, std::char_traits<char>, std::allocator<char> > , the compiler is just expanding the typedef in the error message.

However, why the code is not working I don't know. You appear to have cut out part of your error message where the ' is.

Answer 4

You need to add a const qualification to getAutherName to allow it to be called on const Song objects:

//| -- Returns a `const` `string` (this is bad practice
//v    because it inhibits optimization)
const string getAutherName() const;
// Can be called on a `const`    ^
// `Song`. This is good practice |
// because it it allows `getAutherName()`
// to be used in more places, and it encourages
// const-correctness.