Question

I try to convert uint64_t to const char *. But I find If I use
const char *sz2 = std::to_string(channel_id2).c_str();, it print nothing.

when I use string to take std::to_string(channel_id2) s result and convert the string to const char *, it can print the infomation normally. Then I make other experiments.

Answer 1

This is fine:

// This creates a temp string
std::string temp = std::to_string(channel_id2);

// This is fine, because temp is a variable (and so exists in memory)
const char *sz2 = temp.c_str();

然而,如果是的话:

const char *sz2 = std::to_string(channel_id2).c_str();

行动顺序是:

Call std::to_string, which generates a temporary string object.
Use c_str() to get a pointer to the temporary string contents
Free the temporary string (thus freeing the memory that sz2 is pointing too)

其结果是,sz2无所作为,因此,你获得了不实的印本。

或者说要把你的一条法典跨过多个方面,这就是:

const char *sz2; //< uninitialised

// scope the lifespan of the temporary
{
  // generate temp
  std::string temp = std::to_string(channel_id2);

  // grab address..
  sz2 = temp.c_str();

  // at this point, the destructor of temp is called... 
}

// now sz2 is invalid from here

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