Connect 2+ strings to 1 string

Question

When I try to connect string objects with other strings i'll get a "basic_string" which is not accepted by my method.

I got the following error:

error: no matching function for call to ‘Class1::doSomething(std::__cxx11::basic_string)’

My main class:

using namespace std;
int main() {
    string s = "test";
    Class1* class1 = new Class1();
    class1->doSomething("some text " + s + " too");
}

My "class1":

using namespace std;
class Class1 {
     public:
         void doSomething(const char* sql) { 
              mysqli_query(connection, sql);
         }
}

Why I can't just "+" the strings together?

_"`Class1 class1 = new Class1();`"_ You meant `Class1* class1 = new Class1();`, right? Otherwise this doesn't compile for a different reason. But better to drop that dynamic allocation altogether..... — Lightness Races in Orbit, Apr 03 '18 at 21:21

Lightness Races in Orbit · Accepted Answer · 2018-04-03T21:17:47.080

3

You can.

The error is not related to the +.

The error is because you are passing a std::string to a function that wants a const char*, and the two things are not the same.

Fixable, though, because std::string has a member function c_str() that gives you a C-style string out of its contents, precisely for this kind of occasion:

class1->doSomething(("some text " + s + " too").c_str());

As an aside, a minimal example would have been:

std::string str = "hi";
Class1 class1;
class1.doSomething(str);

Then you would have known already that the + were unrelated.

If Class1 is yours, you may find it more idiomatic to allow your class to accept a C++ string, only "converting" for the MySQL API when necessary:

void doSomething(const std::string& sql) {
    doSomething(sql.c_str());
}

void doSomething(const char* sql) { 
    mysqli_query(connection, sql);
}

(I've kept the original overload because if you originally had a C-string, constructing a std::string just for this function call would be wasteful.)

Now your original usage is fine.

edited Apr 03 '18 at 21:17

answered Apr 03 '18 at 21:15

Lightness Races in Orbit

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I'd rather change signature of `Class1::doSomething` to accept `std::string` and use `sql.c_str()` in the method instead. Also, I'd note that [using namespace `std` is bad practice](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice). – Zereges Apr 03 '18 at 21:17
@Zereges: Jinx! :) – Lightness Races in Orbit Apr 03 '18 at 21:17
@Zereges could be a pessimization. – SergeyA Apr 03 '18 at 21:19
I would say, `std::string_view` :) – SergeyA Apr 03 '18 at 21:19
@SergeyA: Okay, fair enough, that's a good call. I have heard of that but am not a C++17er (or C++20, whatever) – Lightness Races in Orbit Apr 03 '18 at 21:19
LRiO, not the way you've done it, but this is not how I understood Zerges's suggestion. – SergeyA Apr 03 '18 at 21:19

Connect 2+ strings to 1 string

1 Answers1