feedc0de/boost_optional
1
0
Fork 0
forked from boostorg/optional
Code Pull Requests Activity

various documentation fixes

Browse Source
This commit is contained in:
Andrzej Krzemienski
2014-06-20 16:25:57 +02:00
parent 4cbb67e505
commit 599c75a6d3
7 changed files with 211 additions and 222 deletions
Download Patch File Download Diff File Expand all files Collapse all files

25
doc/01_quick_start.qbk
View File

@@ -17,18 +17,18 @@
Let's write and use a converter function that converts an a `std::string` to an `int`. It is possible that for a given string (e.g. `"cat"`) there exist no value of type `int` capable of representing the conversion result. We do not consider such situation an error. We expect that the converter can be used only to check if the conversion is possible. A natural signature for this function can be:
#include <boost/optional.hpp>
boost::optionl<int> convert(const std::string& text);
boost::optional<int> convert(const std::string& text);
All necessary functionality can be included with one header `<boost/optional.hpp>`. The above function signature means that the function can either return a value of type `int` or a flag indicating that no value of `int` is available. This does not indicate an error. It is like one additional value of `int`. This is how we can use our function:
const std::string& text = /*... */;
boost::optionl<int> oi = convert(text); // move-construct
if (oi) // contextual conversion to bool
int i = *oi; // operator*
boost::optional<int> oi = convert(text); // move-construct
if (oi) // contextual conversion to bool
int i = *oi; // operator*
In order to test if `optional` contains a value, we use the contextual conversion to type `bool`. Because of this we can combine the initialization of the optional object and the test into one instruction:
if (boost::optionl<int> oi = convert(text))
if (boost::optional<int> oi = convert(text))
int i = *oi;
We extract the contained value with `operator*` (and with `operator->` where it makes sense). An attempt to extract the contained value of an uninitialized optional object is an ['undefined behaviour] (UB). This implementation guards the call with `BOOST_ASSERT`. Therefore you should be sure that the contained value is there before extracting. For instance, the following code is reasonably UB-safe:
@@ -50,16 +50,19 @@ This version throws an exception upon an attempt to access a non-existent contai
This uses the `atoi`-like approach to conversions: if `text` does not represent an integral number just return `0`. Finally, you can provide a callback to be called when trying to access the contained value fails:
int l = convert(text).value_or_eval([]() -> int {
cout << "could not convert; using -1 instead" << endl;
int fallback_to_default()
{
cerr << "could not convert; using -1 instead" << endl;
return -1;
});
}
int l = convert(text).value_or_eval(fallback_to_default);
This will call the provided callback and return whatever the callback returns. The callback can have side effects: they will only be observed when the optional object does not contain a value.
Now, let's consider how function `convert` can be implemented.
boost::optionl<int> convert(const std::string& text)
boost::optional<int> convert(const std::string& text)
{
std::stringstream s(text);
int i;
@@ -77,9 +80,9 @@ Observe the two return statements. `return i` uses the converting constructor th
We could write function `convert` in a slightly different manner, so that it has a single `return`-statement:
boost::optionl<int> convert(const std::string& text)
boost::optional<int> convert(const std::string& text)
{
boost::optionl<int> ans;
boost::optional<int> ans;
std::stringstream s(text);
int i;
if ((s >> i) && s.get() == std::char_traits<char>::eof())