const引用返回的stl字符串与const char *

Question

i have a STL string member variable in my class,

class A {
public:
   const char* getData1() const { return data.c_str());
   const string& getData2() const { return _data; }
private:
   string _data;
};

getData1() vs. getData2() , which one is better?

Answer 1

The one returning a string reference is the better of the two choices. If the string is changed internally, a previously returned string reference would still be valid, but the const char * probably would not be.

Better than either of those choices may be to just return by value.

std::string getData3() const { return data_; }

Also, I'd suggest not prefixing variables with an underscore, and get rid of that using namespace std; statement.

Answer 2

Both are a little dangerous because your object may go out of scope, or its memory may be freed. But you'll still have a pointer or reference to one of its members.

If you really want to do this though, it just depends on if you want access to the string methods or not. Also c_str() 's const char* may be memory that will be freed on you.

If _data is not that big maybe just return a string , not a const string& .

Answer 3

Generally I prefer getData2() . The const char* returned by c_str() method can only be used as long as data 's non-const methods are not called. After that, the content of const char* is undefined. By returning const char* , you are masking this issue. If the user of the class needs to use a C-style string, he can call c_str() himself on the returned const string& .

Here we are also assuming that the user of getData2() reasonably assumes that the returned reference will only available to him as long as the object of class A is not destroyed yet.

Answer 4

Returning a const string& is better. If you need a const char* , you can just call getData2().c_str() which is practically identical to what getData1() does.

Remember std::string s are better than const char* strings because they store the length, instead of being null terminated. This means if you want to get the length of that string, strlen(getData1()) is an O(n) operation, but getData2().length() is an O(1) operation.