Temporarily in function call: UB?

Question

Consider the following code based on this answer :

#include <iostream>
#include <sstream>

class StringBuilder {
public:
    template <typename T> inline StringBuilder &operator<<(T const &t) {
        ss << t;
        return *this;
    }

    inline char const * c_str() {
        return ss.str().c_str();
    }

private:
    std::stringstream ss;
};

void foo(const char *x) {
    std::cout << x << std::endl;
}

int main() {
    foo((StringBuilder() << "testing " << 12 << 3 << 4).c_str());
    return 0;
}

Does a call foo()with a temporary return value StringBuilderallow UB to be called?

The reason why I ask is that the above example works fine, but in real life I use a library that, among other things, contains logging tools, and using this library, I get the wrong output (logging function accepts mine is char*correct, but it overwrites it internally, which made me believe that the memory is no longer valid).

Answer 1

, - , , , .

StringBuilder , foo, .

c_str() .str().c_str(), , str(), StringBuilder::c_str(), , . UB.