. Doing so avoids (relatively) expensive malloc/free calls, which dwarf the
cost of adding the elements to the container. </p>
+</div>
+
+
<!-- ======================================================================= -->
<h3>
<a name="ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
<div>
There are a variety of sequential containers available for you, based on your
needs. Pick the first in this section that will do what you want.
-
+
<!-- _______________________________________________________________________ -->
<h4>
<a name="dss_arrayref">llvm/ADT/ArrayRef.h</a>
</div>
-<div>
-
<!-- _______________________________________________________________________ -->
<h4>
<a name="dss_smallvector">"llvm/ADT/SmallVector.h"</a>
underlying container but don't affect the cost of the container itself.</p>
</div>
-
</div>
<!-- ======================================================================= -->
<div>
<p>
-TODO: const char* vs stringref vs smallstring vs std::string. Describe twine,
-xref to #string_apis.
+There are a variety of ways to pass around and use strings in C and C++, and
+LLVM adds a few new options to choose from. Pick the first option on this list
+that will do what you need, they are ordered according to their relative cost.
+</p>
+<p>
+Note that is is generally preferred to <em>not</em> pass strings around as
+"<tt>const char*</tt>"'s. These have a number of problems, including the fact
+that they cannot represent embedded nul ("\0") characters, and do not have a
+length available efficiently. The general replacement for '<tt>const
+char*</tt>' is StringRef.
</p>
+
+<p>For more information on choosing string containers for APIs, please see
+<a href="#string_apis">Passing strings</a>.</p>
+
+
+<!-- _______________________________________________________________________ -->
+<h4>
+ <a name="dss_stringref">llvm/ADT/StringRef.h</a>
+</h4>
+<div>
+<p>
+The StringRef class is a simple value class that contains a pointer to a
+character and a length, and is quite related to the <a
+href="#dss_arrayref">ArrayRef</a> class (but specialized for arrays of
+characters). Because StringRef carries a length with it, it safely handles
+strings with embedded nul characters in it, getting the length does not require
+a strlen call, and it even has very convenient APIs for slicing and dicing the
+character range that it represents.
+</p>
+
+<p>
+StringRef is ideal for passing simple strings around that are known to be live,
+either because they are C string literals, std::string, a C array, or a
+SmallVector. Each of these cases has an efficient implicit conversion to
+StringRef, which doesn't result in a dynamic strlen being executed.
+</p>
+
+<p>StringRef has a few major limitations which make more powerful string
+containers useful:</p>
+
+<ol>
+<li>You cannot directly convert a StringRef to a 'const char*' because there is
+no way to add a trailing nul (unlike the .c_str() method on various stronger
+classes).</li>
+
+
+<li>StringRef doesn't own or keep alive the underlying string bytes.
+As such it can easily lead to dangling pointers, and is not suitable for
+embedding in datastructures in most cases (instead, use an std::string or
+something like that).</li>
+
+<li>For the same reason, StringRef cannot be used as the return value of a
+method if the method "computes" the result string. Instead, use
+std::string.</li>
+
+<li>StringRef's allow you to mutate the pointed-to string bytes, but because it
+doesn't own the string, it doesn't allow you to insert or remove bytes from
+the range. For editing operations like this, it interoperates with the
+<a href="#dss_twine">Twine</a> class.</li>
+</ol>
+
+<p>Because of its strengths and limitations, it is very common for a function to
+take a StringRef and for a method on an object to return a StringRef that
+points into some string that it owns.</p>
+
</div>
-
+
+<!-- _______________________________________________________________________ -->
+<h4>
+ <a name="dss_twine">llvm/ADT/Twine.h</a>
+</h4>
+
+<div>
+ <p>
+ The Twine class is used as an intermediary datatype for APIs that want to take
+ a string that can be constructed inline with a series of concatenations.
+ Twine works by forming recursive instances of the Twine datatype (a simple
+ value object) on the stack as temporary objects, linking them together into a
+ tree which is then linearized when the Twine is consumed. Twine is only safe
+ to use as the argument to a function, and should always be a const reference,
+ e.g.:
+ </p>
+
+ <pre>
+ void foo(const Twine &T);
+ ...
+ StringRef X = ...
+ unsigned i = ...
+ foo(X + "." + Twine(i));
+ </pre>
+
+ <p>This example forms a string like "blarg.42" by concatenating the values
+ together, and does not form intermediate strings containing "blarg" or
+ "blarg.".
+ </p>
+
+ <p>Because Twine is constructed with temporary objects on the stack, and
+ because these instances are destroyed at the end of the current statement,
+ it is an inherently dangerous API. For example, this simple variant contains
+ undefined behavior and will probably crash:</p>
+
+ <pre>
+ void foo(const Twine &T);
+ ...
+ StringRef X = ...
+ unsigned i = ...
+ const Twine &Tmp = X + "." + Twine(i);
+ foo(Tmp);
+ </pre>
+
+ <p>... because the temporaries are destroyed before the call. That said,
+ Twine's are much more efficient than intermediate std::string temporaries, and
+ they work really well with StringRef. Just be aware of their limitations.</p>
+
+</div>
+
+
+<!-- _______________________________________________________________________ -->
+<h4>
+ <a name="dss_smallstring">llvm/ADT/SmallString.h</a>
+</h4>
+
+<div>
+
+<p>SmallString is a subclass of <a href="#dss_smallvector">SmallVector</a> that
+adds some convenience APIs like += that takes StringRef's. SmallString avoids
+allocating memory in the case when the preallocated space is enough to hold its
+data, and it calls back to general heap allocation when required. Since it owns
+its data, it is very safe to use and supports full mutation of the string.</p>
+
+<p>Like SmallVector's, the big downside to SmallString is their sizeof. While
+they are optimized for small strings, they themselves are not particularly
+small. This means that they work great for temporary scratch buffers on the
+stack, but should not generally be put into the heap: it is very rare to
+see a SmallString as the member of a frequently-allocated heap data structure
+or returned by-value.
+</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<h4>
+ <a name="dss_stdstring">std::string</a>
+</h4>
+
+<div>
+
+ <p>The standard C++ std::string class is a very general class that (like
+ SmallString) owns its underlying data. sizeof(std::string) is very reasonable
+ so it can be embedded into heap data structures and returned by-value.
+ On the other hand, std::string is highly inefficient for inline editing (e.g.
+ concatenating a bunch of stuff together) and because it is provided by the
+ standard library, its performance characteristics depend a lot of the host
+ standard library (e.g. libc++ and MSVC provide a highly optimized string
+ class, GCC contains a really slow implementation).
+ </p>
+
+ <p>The major disadvantage of std::string is that almost every operation that
+ makes them larger can allocate memory, which is slow. As such, it is better
+ to use SmallVector or Twine as a scratch buffer, but then use std::string to
+ persist the result.</p>
+
+
+</div>
+
+<!-- end of strings -->
+</div>
+
<!-- ======================================================================= -->
<h3>
projects / oota-llvm.git / commitdiff