comparing c++ string

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?

std::basic_string<... just is the actual thing used in the background. The error is still about your usage of std::string. — stefan
– stefan, Commented Jan 18, 2013 at 14:49
@MarounMaroun - this is C++, == is overloaded and allows for string equality comparisons. — wkl
– wkl, Commented Jan 18, 2013 at 14:49
std::string is just a typedef for std::basic_string<char> — cschwan
– cschwan, Commented Jan 18, 2013 at 14:50
Please copy the exact error message, what you have there looks strange. Also getAutherName() isn't const, you can't call with those Song references AFAICT. — Mat
– Mat, Commented Jan 18, 2013 at 14:52

Angew is no longer proud of SO · Accepted Answer · 2013-01-18 14:52:32Z

5

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;
}

answered Jan 18, 2013 at 14:52

Angew is no longer proud of SO

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3 Comments

Steve Over a year ago

Also, it should probably be returning const string &.

Angew is no longer proud of SO Over a year ago

@Steve Depends on whether the string is stored or computed. With C++11's move semantics, I'd be more inklined to return just string. Not const string, though.

Steve Over a year ago

I'd guess that it's returning a private member so the copy in the return can't be elided (I think) so a reference is likely better in this case. But yeah, just const string is probably wrong.

juanchopanza · Accepted Answer · 2013-01-18 14:53:06Z

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;

answered Jan 18, 2013 at 14:53

juanchopanza

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Comments

Robᵩ · Accepted Answer · 2013-01-18 15:05:35Z

1

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.

edited Jan 18, 2013 at 15:05

Robᵩ

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answered Jan 18, 2013 at 14:51

Steve

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1 Comment

Steve Over a year ago

Odd, but as others have said, the problem is the method not being declared as const.

Mankarse · Accepted Answer · 2013-01-18 22:53:00Z

0

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.

answered Jan 18, 2013 at 22:53

Mankarse

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