Why is this assert failing?

Question

The assert in main.cpp is failing, and I don't understand why. Here is string.hpp

class String
{
private:
    int len;
    char* str;
public:
    String(char const* s);      // C-string constructor
    ~String() {delete str;};    // destructor
    char* const getString();    //get string for printing
};

inline bool operator==(String lhs, String rhs)
{
    return std::strcmp(lhs.getString(),rhs.getString());
}

// Define operator!= in terms of ==
inline bool operator!=(String const& lhs, String const& rhs)
{
    return !(lhs == rhs);
}

and here is string.cpp

String::String(char const* s)   // C-string constructor
{
    len = std::strlen(s);
    str = new char[len+1];
    std::strcpy(str,s);

}

char* const String::getString()
{
    return str;
}

and here is main.cpp

#include <cassert>
int main()
{
    String c = "c";
    String d = "d";

    assert(c == c);
    assert(c != d);
}

I tried to include only the essential code. I left out a lot of the obvious includes. The assert(c == d) is failing and I don't understand why. The operator overload of == should have returned a true result.

You need to follow the [Rule of Three](http://stackoverflow.com/q/4172722/241631). Your `operator==` takes the `String`s by value which will result in double deletion otherwise. — Praetorian, Oct 21 '15 at 23:40
Unrelated to the problem at hand: You memory management is horribly wrong. There is also no need to pass the arguments of comparison operators by value, use `const&` instead. — Baum mit Augen, Oct 21 '15 at 23:42
@LeonhartDan http://stackoverflow.com/questions/4172722/what-is-the-rule-of-three has an example almost identical to yours. — Baum mit Augen, Oct 21 '15 at 23:45

score 1 · Answer 1 · answered Oct 21 '15 at 23:40

1

std::strcmp returns 0 if the strings are equal. So your operator== will return false for equal strings and true else.

You could, for instance, switch the implementations of == and != around,

answered Oct 21 '15 at 23:40

Baum mit Augen

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Emil Laine · Accepted Answer · 2015-10-21T23:44:23.937

0

strcmp returns 0 when its arguments have equal contents.

So add a comparison with 0 to your operator==:

inline bool operator==(String const& lhs, String const& rhs)
{
    return std::strcmp(lhs.getString(), rhs.getString()) == 0;
}

Also, since you probably don't want to copy the arguments each time you call operator==, I'd recommend passing them by reference.

edited Oct 21 '15 at 23:44

answered Oct 21 '15 at 23:40

Emil Laine

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Why is this assert failing?

2 Answers2