blob: dfa83b39b5e29cc900c4c08e5cfb97c03e994366 [file] [log] [blame]
.. _module-pw_string-guide:
====================
Get Started & Guides
====================
.. pigweed-module-subpage::
:name: pw_string
:tagline: pw_string: Efficient, easy, and safe string manipulation
.. _module-pw_string-get-started:
Get Started
===========
.. tab-set::
.. tab-item:: Bazel
Add ``@pigweed//pw_string`` to the ``deps`` list in your Bazel target:
.. code-block::
cc_library("...") {
# ...
deps = [
# ...
"@pigweed//pw_string",
# ...
]
}
If only one part of the module is needed, depend only on it; for example
``@pigweed//pw_string:format``.
This assumes ``@pigweed`` is the name you pulled Pigweed into your Bazel
``WORKSPACE`` as.
.. tab-item:: GN
Add ``$dir_pw_string`` to the ``deps`` list in your ``pw_executable()``
build target:
.. code-block::
pw_executable("...") {
# ...
deps = [
# ...
"$dir_pw_string",
# ...
]
}
See `//source/BUILD.gn <https://pigweed.googlesource.com/pigweed/sample_project/+/refs/heads/main/source/BUILD.gn>`_
in the Pigweed Sample Project for an example.
.. tab-item:: CMake
Add ``pw_string`` to your ``pw_add_library`` or similar CMake target:
.. code-block::
pw_add_library(my_library STATIC
HEADERS
...
PRIVATE_DEPS
# ...
pw_string
# ...
)
For a narrower dependency, depend on subtargets like
``pw_string.builder``, etc.
.. tab-item:: Zephyr
There are two ways to use ``pw_string`` from a Zephyr project:
#. Depend on ``pw_string`` in your CMake target (see CMake tab). This is
Pigweed Team's suggested approach since it enables precise CMake
dependency analysis.
#. Add ``CONFIG_PIGWEED_STRING=y`` to the Zephyr project's configuration,
which causes ``pw_string`` to become a global dependency and have the
includes exposed to all targets. Pigweed team does not recommend this
approach, though it is the typical Zephyr solution.
Choose between pw::InlineString and pw::StringBuilder
=====================================================
`pw::InlineString` is intended to replace typical null terminated character
arrays in embedded data structures. Use :cpp:type:`pw::InlineString` if you
need:
* Compatibility with ``std::string``
* Storage internal to the object
* A string object to persist in other data structures
* Lower code size overhead
`pw::StringBuilder` is intended to ease constructing strings in external data;
typically created on the stack and disposed of in the same function. Use
:cpp:class:`pw::StringBuilder` if you need:
* Compatibility with ``std::ostringstream``, including custom object support
* Storage external to the object
* Non-fatal handling of failed append/format operations
* Tracking of the status of a series of operations
* A temporary stack object to aid string construction
* Medium code size overhead
An example of when to prefer :cpp:type:`pw::InlineString` is wrapping a
length-delimited string (e.g. ``std::string_view``) for APIs that require null
termination:
.. code-block:: cpp
#include <string>
#include "pw_log/log.h"
#include "pw_string/string_builder.h"
void ProcessName(std::string_view name) {
// %s format strings require null terminated strings, so create one on the
// stack with size up to kMaxNameLen, copy the string view `name` contents
// into it, add a null terminator, and log it.
PW_LOG_DEBUG("The name is %s",
pw::InlineString<kMaxNameLen>(name).c_str());
}
An example of when to prefer :cpp:class:`pw::StringBuilder` is when
constructing a string for external use.
.. code-block:: cpp
#include "pw_string/string_builder.h"
pw::Status FlushSensorValueToUart(int32_t sensor_value) {
pw::StringBuffer<42> sb;
sb << "Sensor value: ";
sb << sensor_value; // Formats as int.
FlushCStringToUart(sb.c_str());
if (!sb.status().ok) {
format_error_metric.Increment(); // Track overflows.
}
return sb.status();
}
.. _module-pw_string-guide-stringbuilder:
Build a string with pw::StringBuilder
=====================================
The following shows basic use of a :cpp:class:`pw::StringBuilder`.
.. code-block:: cpp
#include "pw_log/log.h"
#include "pw_string/string_builder.h"
pw::Status LogProducedData(std::string_view func_name,
span<const std::byte> data) {
// pw::StringBuffer allocates a pw::StringBuilder with a built-in buffer.
pw::StringBuffer<42> sb;
// Append a std::string_view to the buffer.
sb << func_name;
// Append a format string to the buffer.
sb.Format(" produced %d bytes of data: ", static_cast<int>(data.data()));
// Append bytes as hex to the buffer.
sb << data;
// Log the final string.
PW_LOG_DEBUG("%s", sb.c_str());
// Errors encountered while mutating the string builder are tracked.
return sb.status();
}
Build a string with pw::InlineString
====================================
:cpp:type:`pw::InlineString` objects must be constructed by specifying a fixed
capacity for the string.
.. code-block:: c++
#include "pw_string/string.h"
// Initialize from a C string.
pw::InlineString<32> inline_string = "Literally";
inline_string.append('?', 3); // contains "Literally???"
// Supports copying into known-capacity strings.
pw::InlineString<64> other = inline_string;
// Supports various helpful std::string functions
if (inline_string.starts_with("Lit") || inline_string == "not\0literally"sv) {
other += inline_string;
}
// Like std::string, InlineString is always null terminated when accessed
// through c_str(). InlineString can be used to null-terminate
// length-delimited strings for APIs that expect null-terminated strings.
std::string_view file(".gif");
if (std::fopen(pw::InlineString<kMaxNameLen>(file).c_str(), "r") == nullptr) {
return;
}
// pw::InlineString integrates well with std::string_view. It supports
// implicit conversions to and from std::string_view.
inline_string = std::string_view("not\0literally", 12);
FunctionThatTakesAStringView(inline_string);
FunctionThatTakesAnInlineString(std::string_view("1234", 4));
Build a string inside an pw::InlineString with a pw::StringBuilder
==================================================================
:cpp:class:`pw::StringBuilder` can build a string in a
:cpp:type:`pw::InlineString`:
.. code-block:: c++
#include "pw_string/string.h"
void DoFoo() {
InlineString<32> inline_str;
StringBuilder sb(inline_str);
sb << 123 << "456";
// inline_str contains "456"
}
Pass an pw::InlineString object as a parameter
==============================================
:cpp:type:`pw::InlineString` objects can be passed to non-templated functions
via type erasure. This saves code size in most cases, since it avoids template
expansions triggered by string size differences.
Unknown size strings
--------------------
To operate on :cpp:type:`pw::InlineString` objects without knowing their type,
use the ``pw::InlineString<>`` type, shown in the examples below:
.. code-block:: c++
// Note that the first argument is a generically-sized InlineString.
void RemoveSuffix(pw::InlineString<>& string, std::string_view suffix) {
if (string.ends_with(suffix)) {
string.resize(string.size() - suffix.size());
}
}
void DoStuff() {
pw::InlineString<32> str1 = "Good morning!";
RemoveSuffix(str1, " morning!");
pw::InlineString<40> str2 = "Good";
RemoveSuffix(str2, " morning!");
PW_ASSERT(str1 == str2);
}
However, generically sized :cpp:type:`pw::InlineString` objects don't work in
``constexpr`` contexts.
Known size strings
------------------
:cpp:type:`pw::InlineString` operations on known-size strings may be used in
``constexpr`` expressions.
.. code-block:: c++
static constexpr pw::InlineString<64> kMyString = [] {
pw::InlineString<64> string;
for (int i = 0; i < 10; ++i) {
string += "Hello";
}
return string;
}();
Initialization of pw::InlineString objects
===========================================
:cpp:type:`pw::InlineBasicString` supports class template argument deduction
(CTAD) in C++17 and newer. Since :cpp:type:`pw::InlineString` is an alias, CTAD
is not supported until C++20.
.. code-block:: c++
// Deduces a capacity of 5 characters to match the 5-character string literal
// (not counting the null terminator).
pw::InlineBasicString inline_string = "12345";
// In C++20, CTAD may be used with the pw::InlineString alias.
pw::InlineString my_other_string("123456789");
Custom types with pw::StringBuilder
===================================
As with ``std::ostream``, pw::StringBuilder supports printing custom types by
overriding the ``<<`` operator. This is is done by defining ``operator<<`` in
the same namespace as the custom type. For example:
.. code-block:: cpp
namespace my_project {
struct MyType {
int foo;
const char* bar;
};
pw::StringBuilder& operator<<(pw::StringBuilder& sb, const MyType& value) {
return sb << "MyType(" << value.foo << ", " << value.bar << ')';
}
} // namespace my_project
Internally, ``StringBuilder`` uses the ``ToString`` function to print. The
``ToString`` template function can be specialized to support custom types with
``StringBuilder``, though it is recommended to overload ``operator<<`` instead.
This example shows how to specialize ``pw::ToString``:
.. code-block:: cpp
#include "pw_string/to_string.h"
namespace pw {
template <>
StatusWithSize ToString<MyStatus>(MyStatus value, span<char> buffer) {
return Copy(MyStatusString(value), buffer);
}
} // namespace pw