如何将本地std :: string复制到const char *？

Question

const char* getString() {
  std::string myString = "Hello!";
  return myString.str().c_str();
}

How can I get this function to return a const char * which will live past the local scope in which it is declared? I believe I need to use malloc , but I'm not sure.

Answer 1

我想你需要一个string s的静态查找表，以便SymbolLookUp可以返回c_str值。

Answer 2

It looks like SymbolLookup API you're hooking into is assuming that the symbols are stored in a quasi-permanent structure and not generated on the fly. If your symbols are really transient, you need to make them permanent in some way, either C style through something like

return strdup(myString.c_str());

or in a more idiomatic C++ style with:

static std::vector<std::string> sStringPool;
sStringPool.push_back(myString);
return sStringPool.back().c_str();

Naturally, this is going to lead to unbounded memory growth, but if you have no other information about string lifetimes, there are few alternatives. If you want to get clever, you can at least unique the strings:

static std::set<std::string> sStringPool;
return sStringPool.insert(sStringPool.end(), myString)->c_str();

Answer 3

Simply return the std::string and keep using it.

Just be careful about its lifetime, for example

const char* s = getString().c_str(); doStuff(s);

will cause the second statement to fail because the temporary string returned by getString only lives until the end of the statement.

If the API you are using takes a const char* , the following would work:

callTheAPI(getString().c_str());

and that string would then live for until the API returns.

Answer 4

EDIT: note, that it answers the question as it was stated. At time of this edit it seem obvious the question is misstated wrt original intent, but improved answer can't be provided until consolidation, if it's possible at all.)

Just don't do it. The whole point of using string class is to not use malloc and whatever crap. If you can't provide a const char* to a suitably stable existing location, return std::string from the function (or some other string class matching your taste).

Answer 5

const char* getString() {
  std::string myString = "Hello!";
  return myString.str().c_str();
}

1) The correct syntax is myString.c_str()

2) That function returns a string on stack, so when you return such data, the compiler either makes a copy of it or does some optimization

3) Note that it returns const char * that means the correct way to bind that return value is to have statement such as const char *returnValue = getString() or when you pass that into a function that expects a const char. And because we are assigning a const char * to the returnValue, from C++ spec, that data will live passed the lifetime of the function. So that means we can do const char *str = getString() where getString is your function and it is defined and that the value returned by getString() lives passed the function's lifetime.

I don't see any necessary problem with the code you've posted above.