xtd - Reference Guide
0.1.0
Modern c++17/20 framework to create console, GUI and unit test applications on Windows, macOS, Linux, iOS and android.
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Contains formatting converting documentation.
Formatting is the process of converting an instance of a class, structure, or enumeration value to its string representation, often so that the resulting string can be displayed to users or deserialized to restore the original data type. This conversion can pose a number of challenges:
xtd provides rich formatting support that enables developers to address these requirements.
This overview contains the following sections:
The basic mechanism for formatting is the default implementation of the operator << object method, which is discussed in the DefaultFormattingUsingOperatorShiftLeftSection section later in this topic. xtd.Strings provides several ways to modify and extend its default formatting support. These include the following:
For more information about format specifiers, see the ToStringMethodAndFormatStringsSection section.
For more information about formatting with format providers, see the CultureSensitiveFormattingSection section.
The following sections examine these methods for converting an object to its string representation.
The composite formatting feature takes a list of objects and a composite format string as input. A composite format string consists of fixed text intermixed with indexed placeholders, called format items, that correspond to the objects in the list. The formatting operation yields a result string that consists of the original fixed text intermixed with the string representation of the objects in the list.
The composite formatting feature is supported by methods such as the following:
A composite format string and object list are used as arguments of methods that support the composite formatting feature. A composite format string consists of zero or more runs of fixed text intermixed with one or more format items. The fixed text is any string that you choose, and each format item corresponds to an object or boxed structure in the list. The composite formatting feature returns a new result string where each format item is replaced by the string representation of the corresponding object in the list.
Consider the following Format code fragment.
The fixed text is "Name = " and ", age = ". The format items are "{0}", whose index is 0, which corresponds to the object name, and "{1:D3}", whose index is 1, which corresponds to the integer 42.
Each format item takes the following form and consists of the following components:
The matching braces ("{" and "}") are required.
The optional index component, also called a parameter specifier, is a number starting from 0 that identifies a corresponding item in the list of objects. That is, the format item whose parameter specifier is 0 formats the first object in the list, the format item whose parameter specifier is 1 formats the second object in the list, and so on. The following example includes four parameter specifiers, numbered zero through three, to represent prime numbers less than ten:
Multiple format items can refer to the same element in the list of objects by specifying the same parameter specifier. For example, you can format the same numeric value in hexadecimal, scientific, and number format by specifying a composite format string such as : "0x{0:X} {0:E} {0:N}", as the following example shows.
Each format item can refer to any object in the list. For example, if there are three objects, you can format the second, first, and third object by specifying a composite format string like this: "{1} {0} {2}". An object that is not referenced by a format item is ignored. A std::argument_error is thrown at runtime if a parameter specifier designates an item outside the bounds of the list of objects.
If the index component is not specified, it will be automatically generated in the order of the argument list.
The following example shows format without specified index:
The optional alignment component is a signed integer indicating the preferred formatted field width. If the value of alignment is less than the length of the formatted string, alignment is ignored and the length of the formatted string is used as the field width. The formatted data in the field is right-aligned if alignment is positive and left-aligned if alignment is negative. If padding is necessary, white space is used. The comma is required if alignment is specified.
The following example defines two arrays, one containing the names of employees and the other containing the hours they worked over a two-week period. The composite format string left-aligns the names in a 20-character field, and right-aligns their hours in a 5-character field. Note that the "N1" standard format string is also used to format the hours with one fractional digit.
The optional formatString component is a format string that is appropriate for the type of object being formatted. Specify a numeric format string if the corresponding object is a numeric value, or an enumeration format string if the corresponding object is an enumeration value. If formatString is not specified, the general ("G") format specifier for a numeric or enumeration type is used. The colon is required if formatString is specified.
The following table lists types or categories of types that support a predefined set of format strings, and provides links to the topics that list the supported format strings. Note that string formatting is an extensible mechanism that makes it possible to define new format strings for all existing types as well as to define a set of format strings supported by an application-defined type. For more information, see CustomFormatSection.
Type or Type category | See |
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Boolean type (bool) | BooleanFormatSection |
Date and time type (std::chrono::time_point, std::time_t) | DateTimeFormatSection |
Duration type (std::chrono::duration) | DurationFormatSection |
Enumeration types (all enum or enum class types) | EnumerationFormatSection |
Numeric types (char, unsigned char, short, unsigned short, int, unsigned int, long, unsigned long, long long, unsigned long long, float, double and long double) | Numeric format |
Opening and closing braces are interpreted as starting and ending a format item. Consequently, you must use an escape sequence to display a literal opening brace or closing brace. Specify two opening braces ("{{") in the fixed text to display one opening brace ("{"), or two closing braces ("}}") to display one closing brace ("}"). Braces in a format item are interpreted sequentially in the order they are encountered. Interpreting nested braces is not supported.
The way escaped braces are interpreted can lead to unexpected results. For example, consider the format item "{{{0:D}}}", which is intended to display an opening brace, a numeric value formatted as a decimal number, and a closing brace. However, the format item is actually interpreted in the following manner:
One way to write your code to avoid misinterpreting escaped braces and format items is to format the braces and format item separately. That is, in the first format operation display a literal opening brace, in the next operation display the result of the format item, then in the final operation display a literal closing brace. The following example illustrates this approach.
The following example shows one string created using composite formatting and another created using xtd::to_string method. Both types of formatting produce equivalent results.
xtd::console::write_line exposes the same functionality as xtd::ustring::format. The only difference between the two methods is that xtd::ustring::format returns its result as a string, while xtd::console::write_line writes the result to the output stream (std::cout) associated with the console object. The following example uses the xtd::console::write_line method to format the value of my_int to a currency value.
The following example demonstrates formatting multiple objects, including formatting one object two different ways.
The following example demonstrates the use of alignment in formatting. The arguments that are formatted are placed between vertical bar characters (|) to highlight the resulting alignment.
Standard numeric format strings are used to format common numeric types. A standard numeric format string takes the form Axx, where: