Converting a string to a char*

Question

I have a program where I want to output a list of strings to a console window.

These strings come from my food.getFoodName() method (which returns a string) like "Steak", "Burger", "Sausage" etc.

I'm outputting these strings using a loop. Before I can output these strings, I need to convert the current string in the loop to a const char* so I can use it in my Draw_String() Method.

This is the code concerned in this process:

void DisplayFood(std::vector<Food> foods)
{



    for (int i = 0; i < foods.size(); i++)
    {
        const char * c = foods.at(i).getFoodName().c_str();

        Draw_String(10, i, c);
    }
}

inline void Draw_String(int x, int y, const char *string)
{
    // this function draws a string at the given x,y

    COORD cursor_pos; // used to pass coords
    // set printing position
    cursor_pos.X = x;
    cursor_pos.Y = y;
    SetConsoleCursorPosition(hconsole, cursor_pos);

    // print the string in current color
    printf("%s", string);

} // end Draw_String

std::string Food::getFoodName()
{
    return FoodName;
}

My issue is, this conversion doesn't work as intended, and my output on the screen is basically unreadable ascii symbols like "||||||||||||||||||"

I'm a noob at c++, only been doing this for like 10 weeks. But the issue is either the conversion process (most likely) or the printf method.

Anyone have any idea what I'm doing wrong? I'd appreciate any help.

If you comment out the `SetConsoleCursorPosition` call, do you still get the high ascii output? — Kenney, Nov 07 '15 at 18:30
Irrelevant: `c_str()` returns a `char const*` already. As such, you can directly, `Draw_String(10, i, foods.at(i).getFoodName().c_str());` — 101010, Nov 07 '15 at 18:31
@Andros You have to take care, that the `std::string` instance is still in scope and alive when you use the result of `c_str()`. — πάντα ῥεῖ, Nov 07 '15 at 18:35

score 6 · Accepted Answer · edited May 23 '17 at 12:23

6

foods.at(i).getFoodName()

returns a temporary object, which ends its lifetime after the statement, including the data returned by std::string::c_str. So, accessing the memory pointed to by c is undefined behavior.

Instead, you could

extend the temporary's lifetime by binding it to a const reference:
```
const std::string& foodName = foods.at(i).getFoodName();
```

since C++11, bind it to an rvalue reference:

std::string&& foodName = foods.at(i).getFoodName();

just pass the temporary to the function directly:

Draw_String(10, i, foods.at(i).getFoodName().c_str());

return a reference from Food::getFoodName¹.

You might also have a look at this thread.

Notes:

Why don't you use std::string? std::cout will work fine with it.
std::printf should not be used in usual C++ code.

_{¹ As proposed by @juanchopanza in the comments to this answer}

edited May 23 '17 at 12:23

Community

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answered Nov 07 '15 at 18:35

cadaniluk

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Another option would be to make `getFoodName()` return a `const` reference. – juanchopanza Nov 07 '15 at 18:39
Oh, I see. getfoodName returns a copy. Well spotted. – Peter - Reinstate Monica Nov 07 '15 at 18:40
@juanchopanza Added. Thanks. – cadaniluk Nov 07 '15 at 21:07

Converting a string to a char*

1 Answers1