How to Clear and Reuse an ostringstream?

Reusing an ostringstream

In efforts to optimize resource allocation, one may encounter the need to reset an ostringstream to its initial state to avoid excessive allocations. This article addresses this issue by exploring various methods to clear and reuse an ostringstream.

Resetting the Object

To revert the ostringstream to its original state, a sequence of clear and str can be employed:

<code class="cpp">// Clear any flags (e.g., eof)
s.clear();

// Empty the buffer
s.str("");</code>

This approach effectively clears the buffer and resets the stream pointers. Alternatively, manual clearing can be performed followed by seeking to the beginning:

<code class="cpp">// Clear any flags
s.clear();

// Reset put pointer (for output streams)
s.seekp(0);

// Reset get pointer (for input streams)
s.seekg(0);</code>

This method prevents unnecessary reallocations by overwriting the existing buffer content. For example:

<code class="cpp">std::ostringstream s;
s << "hello";
s.seekp(0);
s << "b";
assert(s.str() == "bello");</code>

To utilize the string for C-style functions, std::ends can be used to append a terminating null character:

<code class="cpp">// Append a terminating null
s << std::ends;
assert(s.str().size() == 5 && std::strlen(s.str().data()) == 1);</code>

Although std::ends is a remnant from the deprecated std::strstream, it remains useful in situations where it's necessary to work with C-style character arrays without explicit null termination.

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