pw_string: Rework Guide in docs

Change-Id: I81045bf83240bba1df6960f7d2a260f6734b9fdf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/133610
Commit-Queue: Keir Mierle <[email protected]>
Reviewed-by: Chad Norvell <[email protected]>

pw_string/guide.rst

@@ -1,11 +1,67 @@
 .. _module-pw_string-guide:
 
-===============
-pw_string guide
-===============
+================
+pw_string: Guide
+================
 
-Building strings with StringBuilder
------------------------------------
+InlineString and StringBuilder?
+===============================
+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
+
+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();
+  }
+
+
+Building strings with pw::StringBuilder
+=======================================
 The following shows basic use of a :cpp:class:`pw::StringBuilder`.
 
 .. code-block:: cpp
@@ -34,92 +90,94 @@ The following shows basic use of a :cpp:class:`pw::StringBuilder`.
     return sb.status();
   }
 
-Constructing pw::InlineString objects
--------------------------------------
+Building strings with pw::InlineString
+======================================
 :cpp:type:`pw::InlineString` objects must be constructed by specifying a fixed
 capacity for the string.
 
 .. code-block:: c++
 
-  // Initialize from a C string.
-  pw::InlineString<32> inline_string = "Literally";
-  inline_string.append('?', 3);   // contains "Literally???"
+   #include "pw_string/string.h"
 
-  // Supports copying into known-capacity strings.
-  pw::InlineString<64> other = inline_string;
+   // Initialize from a C string.
+   pw::InlineString<32> inline_string = "Literally";
+   inline_string.append('?', 3);   // contains "Literally???"
 
-  // Supports various helpful std::string functions
-  if (inline_string.starts_with("Lit") || inline_string == "not\0literally"sv) {
-    other += inline_string;
-  }
+   // Supports copying into known-capacity strings.
+   pw::InlineString<64> 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;
-  }
+   // Supports various helpful std::string functions
+   if (inline_string.starts_with("Lit") || inline_string == "not\0literally"sv) {
+     other += inline_string;
+   }
 
-  // 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);
+   // 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;
+   }
 
-  FunctionThatTakesAStringView(inline_string);
+   // 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);
 
-  FunctionThatTakesAnInlineString(std::string_view("1234", 4));
+   FunctionThatTakesAStringView(inline_string);
 
-Choosing between InlineString and StringBuilder
------------------------------------------------
-:cpp:type:`pw::InlineString` is comparable to ``std::string``, while
-:cpp:class:`pw::StringBuilder` is comparable to ``std::ostringstream``.
-Because :cpp:class:`pw::StringBuilder` provides high-level stream functionality,
-it has more overhead than :cpp:type:`pw::InlineString`.
+   FunctionThatTakesAnInlineString(std::string_view("1234", 4));
 
-Use :cpp:type:`pw::InlineString` unless :cpp:class:`pw::StringBuilder`'s
-capabilities are needed. Features unique to :cpp:class:`pw::StringBuilder`
-include:
+Building strings inside InlineString with a StringBuilder
+=========================================================
+:cpp:class:`pw::StringBuilder` can build a string in a
+:cpp:type:`pw::InlineString`:
 
-* Polymorphic C++ stream-style output, potentially supporting custom types.
-* Non-fatal handling of failed append/format operations.
-* Tracking the status of a series of operations.
-* Building a string in an external buffer.
+.. code-block:: c++
 
-If those features are not required, use :cpp:type:`pw::InlineString`. A common
-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.
+   #include "pw_string/string.h"
 
-.. code-block:: cpp
+   void DoFoo() {
+     InlineString<32> inline_str;
+     StringBuilder sb(inline_str);
+     sb << 123 << "456";
+     // inline_str contains "456"
+   }
 
-  void ProcessName(std::string_view name) {
-    PW_LOG_DEBUG("The name is %s", pw::InlineString<kMaxNameLen>(name).c_str());
+Passing InlineStrings as parameters
+===================================
+: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.
 
-Operating on unknown size strings
----------------------------------
-All :cpp:type:`pw::InlineString` operations may be performed on strings without
-specifying their capacity.
+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++
 
-  void RemoveSuffix(pw::InlineString<>& string, std::string_view suffix) {
-    if (string.ends_with(suffix)) {
-       string.resize(string.size() - suffix.size());
-    }
-  }
+   // 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!");
+   void DoStuff() {
+     pw::InlineString<32> str1 = "Good morning!";
+     RemoveSuffix(str1, " morning!");
 
-    pw::InlineString<40> str2 = "Good";
-    RemoveSuffix(str2, " morning!");
+     pw::InlineString<40> str2 = "Good";
+     RemoveSuffix(str2, " morning!");
 
-    PW_ASSERT(str1 == str2);
-  }
+     PW_ASSERT(str1 == str2);
+   }
+
+However, generically sized :cpp:type:`pw::InlineString` objects don't work in
+``constexpr`` contexts.
 
-Operating on known-size strings
--------------------------------
+Known size strings
+------------------
 :cpp:type:`pw::InlineString` operations on known-size strings may be used in
 ``constexpr`` expressions.
 
@@ -135,13 +193,8 @@ Operating on known-size strings
      return string;
    }();
 
-Building strings
-----------------
-:cpp:class:`pw::StringBuilder` may be used to build a string in a
-:cpp:type:`pw::InlineString`.
-
-Deducing class template arguments with pw::InlineBasicString
-------------------------------------------------------------
+Compact initialization of InlineStrings
+=======================================
 :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.
@@ -155,9 +208,8 @@ is not supported until C++20.
    // In C++20, CTAD may be used with the pw::InlineString alias.
    pw::InlineString my_other_string("123456789");
 
-
-Printing custom types
----------------------
+Supporting custom types with StringBuilder
+==========================================
 As with ``std::ostream``, 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: