内存中分配的 std::string 在哪里？

Question

Here is a function:

void foo() {
   string str = "StackOverflo";
   str.push_back('w');
}

When we declare the string inside the function, is it stored on the Stack or Heap? Why?

string foo() {
   string str = "StackOverflo";
   str.push_back('w');
   return str;
}

Can we return the string reference and continue using somewhere else in the program?

Answer 1

When we declare the String inside the function, is it stored on the Stack or Heap?

The string object itself is stored on the stack but it points to memory that is on the heap.

Why?

The language is defined such that the string object is stored on the stack. string's implementation to construct an object uses memory on the heap.

Answer 2

The object str (it is the instance of the class std::string ) is allocated in the stack. However, the string data itself MAY BE allocated in the heap. It means the object has an internal pointer to a buffer that contains the actual string. However, again, if the string is small (like in this example) usually the string class will have what we call "small string optimization". It means that the size of the std::string object itself is enough to contain the data of the string if it's small enough (usually around 23 bytes + 1 byte for null-terminator)... then if the content is bigger than this, the string data will be allocated in the heap.

Usually you can return your string normally. C++ can handle this for you. The move semantics can take care of whatever necessary here to return the string object pointing to the same string data of the original string, avoiding doing unecessary copies.

Answer 3

"StackOverflo" , the string literal, is likely stored in the read-only data space of the binary and is mapped into memory when the program starts. You can read more about this here .

str , the class instance`, is allocated on the stack. But the memory its constructor allocates to make a copy of the string literal is allocated on the heap.

The foo function returns a copy of str , so what you have coded is ok.

Answer 4

all local variables in C++ are stores in Stack. even local pointers are stored in Stack. but the memory allocated in pointer are stored on Heap.

So, your str is stored in Stack while its value is stored in Heap. because std::string uses char pointer to allocate your "stackoverflow"

Answer 5

On Windows platform, in C++, with std::string, string size less than around 14 char or so (known as small string) is stored in stack with almost no overhead, whereas string size above is stored in heap with overhead. Usually platform and compiler dependent which can be tweaked with optimization options.

std::string myStr {"Hello World"}; // stored in program stack std::string myStr {"StackOverflow saved life of a programmer" }; //pointer is stored in program stack whereas memory for string and is allocated in heap

Below link has good explanation. https://blogs.msmvps.com/gdicanio/2016/11/17/the-small-string-optimization/

Answer 6

When we declare the string inside the function is it stored on the Stack or Heap? Why?

The biggest and maybe the only reason to allocate on the heap is to allow string objects to grow in size, for example by calling push_back() or append() . Objects stored on the stack cannot change its size - that's defined by C/C++ language.