比较字符串对象与C ++中的字符串文字：为什么没有编译错误？

Question

I wrote some code like this:

#include<string>  
using namespace std;  
int main() {  
    string str;
    ...
    if(str=="test")    //valid????
        //do something
    ...
    return 0;
}

After re-reading the code later I was curious that how did the compiler give no errors?
Note: I already checked the reference and it looks like there should be some sort of type mismatch error (comparing string object with array of char)

edit: sorry for the = to == typo error. it is already fixed

edit 2: issues:

• there aren't any of operator==(string,char*) or operator==(string,char[]) or similar operators defined in the reference(cppreference.com)
• no conversion operator from char* or char[] to string

Answer 1

As others have mentioned, the single = sign performs assignment, not comparison.

But the comparison operator is defined, like assignment, by operator overloading , one of the most essential features of C++.

The expression str = "test" is transformed into a function call str.operator= ("test") , and the expression str == "test" would be transformed into either str.operator== ("test") , or operator==(str,"test") , whichever one works.

Even if the overload function weren't defined for operands of std::string and char * , the compiler would still try to find function(s) to convert the arguments to types that did match such a function.

EDIT: Hah, std::string cannot be converted to bool so the if condition would still be an error. I assume this is an artifact of making a nice snippet for the question.

Answer 2

if(str="test")  //it's an assignment not a comparison.

change it to if(str=="test")

 why no compile errors?

Because it's c++ not c. std::string has defined this == operator .

if(str="test")  //it's an error: because you can't convert string to boolean type. 
                  which is expected as condition.

error like :could not convert 's.std::basic_string<_CharT, _Traits, _Alloc>::operator=
<char, std::char_traits<char>, std::allocator<char> >(((const char*)"p"))' from 
'std::basic_string<char>' to 'bool' 

Answer 3

If you do this

if(str="test"){}

you assign "test" to str . As this is a valid operation, the assignment will return &str a reference to your object, so your if-condition will always be fulfilled. Of course, if str == 0 then it will give an error. So better do:

if(str == "test"){}

Thanks to James Kanze!

Answer 4

As mentioned, it isn't compilation error due the operator overloading (ignoring the fact that you're not comparing but assigning). If you use any object without this operator then, yes, it will be a compilation error:

// Foo does not have comparision operator
struct Foo {};

// Bar have comparision operator
struct Bar
{
    // Next line is the operator overload.
    bool operator ==(const char *pstr) const { return true; };
};

// string have comparision operator
std::string Test("test");

if (Foo == "test")  // compilation error
if (Bar == "test")  // uses the Bar::operator ==
if (Test == "test") // uses the basic_string<char>::operator ==
{ /* do something */ }