How do you clear a stringstream variable?

For all the standard library types the member function empty() is a query, not a command, i.e. it means "are you empty?" not "please throw away your contents".

The clear() member function is inherited from ios and is used to clear the error state of the stream, e.g. if a file stream has the error state set to eofbit (end-of-file), then calling clear() will set the error state back to goodbit (no error).

For clearing the contents of a stringstream, using:

m.str("");

is correct, although using:

m.str(std::string());

is technically more efficient, because you avoid invoking the std::string constructor that takes const char*. But any compiler these days should be able to generate the same code in both cases - so I would just go with whatever is more readable.

You can clear the error state and empty the stringstream all in one line

std::stringstream().swap(m); // swap m with a default constructed stringstream

This effectively resets m to a default constructed state, meaning that it actually deletes the buffers allocated by the string stream and resets the error state. Here's an experimental proof:

int main ()
{
    std::string payload(16, 'x');
    
    std::stringstream *ss = new std::stringstream; // Create a memory leak
    (*ss) << payload;                              // Leak more memory
    
    // Now choose a way to "clear" a string stream
    //std::stringstream().swap(*ss); // Method 1
    //ss->str(std::string());        // Method 2
    
    std::cout << "end" << std::endl;
}

Demo

When the demo is compiled with address sanitizer, memory usage is revealed:

=================================================================
==10415==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 392 byte(s) in 1 object(s) allocated from:
    #0 0x510ae8 in operator new(unsigned long) (/tmp/1637178326.0089633/a.out+0x510ae8)
    #1 0x514e80 in main (/tmp/1637178326.0089633/a.out+0x514e80)
    #2 0x7f3079ffb82f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/../csu/libc-start.c:291

Indirect leak of 513 byte(s) in 1 object(s) allocated from:
    #0 0x510ae8 in operator new(unsigned long) (/tmp/1637178326.0089633/a.out+0x510ae8)
    #1 0x7f307b03a25c in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::reserve(unsigned long) (/usr/local/lib64/libstdc++.so.6+0x13725c)
    #2 0x603000000010  (<unknown module>)

SUMMARY: AddressSanitizer: 905 byte(s) leaked in 2 allocation(s).

Pretty steep if you ask me. To hold just 16bytes of payload, we spent 905 bytes ... string streams are no toy. Memory is allocated in two parts:

• The constructed string stream (392 bytes)
• The extra buffer needed for the payload (513 bytes). The extraneous size has to do with the allocation strategy chosen by the stream and for payloads <= 8 bytes, blocks inside the initial object can be used.

If you enable method 1 (the one shown in this answer) the extra 513 (payload) bytes are reclaimed, because the stream is actually cleared.

If you enable method2 as suggested in the comments or other answers, you can see that all 905 bytes are in use by the time we exit.

In terms of program semantics, one may only care that the stream "appears" and "behaves" as empty, similar to how a vector::clear may leave the capacity untouched but render the vector empty to the user (of course vector would spend just 16 bytes here). Given the memory allocation that string stream requires, I can imagine this approach being often faster. This answer's primary goal is to actually clear the string stream, given that memory consumption that comes with it is no joke. Depending on your use case (number of streams, data they hold, frequency of clearing) you may choose the best approach.

Finally note that it's rarely useful to clear the stream without clearing the error state and all inherited state. The one liner in this answer does both.

m.str("");

seems to work.