如何使用std :: string而不复制？

Question

I have a class say,

class Foo
{
   public:
      void ProcessString(std::string &buffer)
      {
          // perform operations on std::string

          // call other functions within class
          // which use same std::string string
      }

      void Bar(std::string &buffer)
      {
          // perform other operations on "std::string" buffer
      }

      void Baz(std::string &buffer)
      {
          // perform other operations on "std::string" buffer
      }
};

This class tries to use a std::string buffer to perform operations on it using various methods under these conditions:

• I don't want to pass a copy of std::string which I already have.
• I don't want to create multiple objects of this class.

For example:

// Once an object is created
Foo myObject;

// We could pass many different std::string's to same method without copying
std::string s1, s2, s3;
myObject.ProcessString(s1);
myObject.ProcessString(s2);
myObject.ProcessString(s3);

I could use the string and assign it as a class member so that other functions using can know about it.

But it seems we cannot have a reference class member std::string &buffer because it can only be initialized from constructor.

I could use a pointer to std::string ie std::string *buffer and use it as a class member and then pass the addresses of s1, s2, s3 .

class Foo
{
   public:
      void ProcessString(std::string *buf)
      {
          // Save pointer
          buffer = buf;

          // perform operations on std::string

          // call other functions within class
          // which use same std::string string
      }

      void Bar()
      {
          // perform other operations on "std::string" buffer
      }

      void Baz()
      {
          // perform other operations on "std::string" buffer
      }
   private:
       std::string *buffer;
};

Or, the other way could be pass each functions a reference to std::string buffer just as shown in the first example above .

Both ways kind of seem a bit ugly workarounds to be able to use a std::string without copying as I have rarely seen the usage of std::string as a pointer or pass all the functions of class the same argument.

Is there a better around this or what I'm doing is just fine?

Answer 1

Keeping in MyObject a reference or a pointer to a string which is not ownned by your object is dangerous. It will be easy to get nasty undefined behaviour .

Look at the following legal example (Bar is public):

myObject.ProcessString(s1);     // start with s1 and keep its address
myObject.Bar();                 // works with s1 (using address previously stored) 

Look at the following UB:

if (is_today) {
    myObject.ProcessString(string("Hello"));  // uses an automatic temporary string
}                                             // !! end of block: temporary is destroyed!
else {
    string tmp = to_string(1234);            // create a block variable 
    myObject.ProcessString(tmp);             // call the main function 
}                                            // !! end of block:  tmp is destroyed
myObject.Bar();  // expects to work with pointer, but in reality use an object that was already destroyed !!  => UB                              

The errors are very nasty, because when reading function's usage, everything seems ok and well managed. The problem is hidden by automatic destruction of bloc variables.

So if you really want to avoid the copy of the string, you could use a pointer as you envisaged, but you shall only use this pointer in functions called directly by ProcessString(), and make these functions private.

In all other case, I'd strongly suggest to reconsider your position, and envisage:

• a local copy of the string in the object that shall use it.
• Or use a string& parameters in all the object's function that need it. This avoids the copies but leaves to caller the responsibility of organising the proper management of the string.

Answer 2

You basically need to answer this question: who owns the string? Does Foo own the string? Does the external caller own the string? Or do they both share ownership of the string.

"Owning" the string means that the lifetime of the string is tied to it. So if Foo owns the string, the string will stop existing when Foo stops existing or destroys it. Shared ownership is far more complicated, but we can make it simpler by saying that the string will exist as long as any of the owners keep it.

Each situation has a different answer:

1. Foo owns the string: Copy the string into Foo , then let the member methods mutate it.
2. External resource owns the string: Foo should never hold a reference to the string outside of its own stack, since the string could be destroyed without its knowledge. This means that it needs to be passed by reference to every method that uses it and does not own it, even if the methods are in the same class.
3. Shared ownership: Use a shared_ptr when creating the string, then pass that shared_ptr to every instance that shares ownership. You then copy the shared_ptr to a member variable, and methods access it. This has much higher overhead then passing by reference, but if you want shared ownership it is one of the safest ways to do so.

There are actually several other kinds of ways to model ownership, but they tend to be more esoteric. Weak ownership, transferable ownership, etc.

Answer 3

Since your requirement is that

1.I don't want to pass a copy of std::string which I already have.

2.I don't want to create multiple objects of this class.

using pass by ref would be the solution to 1 using static would be the solution to 2. since it is a static memeber method, there would be only one copy of this method. it wont belong to any object, though. With that being said, you can call this method directly instead of through an object.

For example,

class Foo
{
      static void ProcessString(std::string &s)
      {
          // perform operations on std::string

          // call other functions within class
          // which use same std::string string
      }

}

when you call this method, it would be something like this:

std::string s1, s2, s3;
Foo::ProcessString(s1);
Foo::ProcessString(s2);
Foo::ProcessString(s3);

One step further, if you want only one instance of this class, you can refer to singleton design pattern.