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105 lines
3.7 KiB
105 lines
3.7 KiB
.. _module-pw_string:
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---------
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pw_string
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---------
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String manipulation is a very common operation, but the standard C and C++
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string libraries have drawbacks. The C++ functions are easy-to-use and powerful,
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but require too much flash and memory for many embedded projects. The C string
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functions are lighter weight, but can be difficult to use correctly. Mishandling
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of null terminators or buffer sizes can result in serious bugs.
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The ``pw_string`` module provides the flexibility, ease-of-use, and safety of
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C++-style string manipulation, but with no dynamic memory allocation and a much
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smaller binary size impact. Using ``pw_string`` in place of the standard C
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functions eliminates issues related to buffer overflow or missing null
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terminators.
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Compatibility
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=============
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C++17
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pw::string::Format
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==================
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The ``pw::string::Format`` and ``pw::string::FormatVaList`` functions provide
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safer alternatives to ``std::snprintf`` and ``std::vsnprintf``. The snprintf
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return value is awkward to interpret, and misinterpreting it can lead to serious
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bugs.
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Size report: replacing snprintf with pw::string::Format
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-------------------------------------------------------
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The ``Format`` functions have a small, fixed code size cost. However, relative
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to equivalent ``std::snprintf`` calls, there is no incremental code size cost to
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using ``Format``.
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.. include:: format_size_report
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pw::StringBuilder
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=================
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``pw::StringBuilder`` facilitates building formatted strings in a fixed-size
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buffer. It is designed to give the flexibility of ``std::string`` and
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``std::ostringstream``, but with a small footprint.
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Supporting custom types with StringBuilder
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------------------------------------------
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As with ``std::ostream``, StringBuilder supports printing custom types by
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overriding the ``<<`` operator. This is is done by defining ``operator<<`` in
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the same namespace as the custom type. For example:
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.. code-block:: cpp
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namespace my_project {
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struct MyType {
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int foo;
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const char* bar;
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};
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pw::StringBuilder& operator<<(pw::StringBuilder& sb, const MyType& value) {
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return sb << "MyType(" << value.foo << ", " << value.bar << ')';
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}
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} // namespace my_project
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Internally, ``StringBuilder`` uses the ``ToString`` function to print. The
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``ToString`` template function can be specialized to support custom types with
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``StringBuilder``, though it is recommended to overload ``operator<<`` instead.
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This example shows how to specialize ``pw::ToString``:
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.. code-block:: cpp
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#include "pw_string/to_string.h"
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namespace pw {
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template <>
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StatusWithSize ToString<MyStatus>(MyStatus value, std::span<char> buffer) {
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return CopyString(MyStatusString(value), buffer);
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}
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} // namespace pw
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Size report: replacing snprintf with pw::StringBuilder
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------------------------------------------------------
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StringBuilder is safe, flexible, and results in much smaller code size than
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using ``std::ostringstream``. However, applications sensitive to code size
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should use StringBuilder with care.
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The fixed code size cost of StringBuilder is significant, though smaller than
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``std::snprintf``. Using StringBuilder's << and append methods exclusively in
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place of ``snprintf`` reduces code size, but ``snprintf`` may be difficult to
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avoid.
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The incremental code size cost of StringBuilder is comparable to ``snprintf`` if
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errors are handled. Each argument to StringBuilder's ``<<`` expands to a
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function call, but one or two StringBuilder appends may have a smaller code size
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impact than a single ``snprintf`` call.
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.. include:: string_builder_size_report
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Future work
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===========
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* StringBuilder's fixed size cost can be dramatically reduced by limiting
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support for 64-bit integers.
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* Consider integrating with the tokenizer module.
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