Save away ostringreader.str().c_str()

Question

Why does the value of text1 change after reading out y in this piece of code?

void func()
{
    int value1 = 5;
    double value2 = 1.5;
    std::ostringstream x, y;

    x << value1;
    y << value2;


    const char *text1 = x.str().c_str();
    fprintf(stderr, "text1: v=%s, p=%p\n", text1, &text1);
    const char *text2 = y.str().c_str();
    fprintf(stderr, "text1: v=%s, p=%p\ntext2: v=%s, p=%p\n", text1, &text1, text2, &text2);
}

Output:

text1: v = 5, a = 0xbfcfd508

text1: v = 1.5, a = 0xbfcfd508

text2: v = 1.5, a = 0xbfcfd510

score 8 · Accepted Answer · answered Apr 25 '12 at 12:51

8

After you have evaluated the str().c_str() expression, the temporary std::string instance created by the call to str() is released and your char pointer points into Nirvana. You need to store the return value of str()!

answered Apr 25 '12 at 12:51

Konrad Rudolph

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1

the moral of the story : avoid c_str(), specially of a temporary – BЈовић Apr 25 '12 at 12:54
3

The return value of [c_str](http://www.cplusplus.com/reference/string/string/c_str/) is only guaranteed valid until the next non-const operation on the object. The destructor is non-const. So you're storing a result that becomes invalid before you use it. – David Schwartz Apr 25 '12 at 12:54

Save away ostringreader.str().c_str()

1 Answers1