forked from catchorg/Catch2
133 lines
4.7 KiB
Markdown
133 lines
4.7 KiB
Markdown
<a id="top"></a>
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# String conversions
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**Contents**<br>
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[operator << overload for std::ostream](#operator--overload-for-stdostream)<br>
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[Catch::StringMaker specialisation](#catchstringmaker-specialisation)<br>
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[Catch::is_range specialisation](#catchis_range-specialisation)<br>
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[Exceptions](#exceptions)<br>
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[Enums](#enums)<br>
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[Floating point precision](#floating-point-precision)<br>
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Catch needs to be able to convert types you use in assertions and logging expressions into strings (for logging and reporting purposes).
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Most built-in or std types are supported out of the box but there are two ways that you can tell Catch how to convert your own types (or other, third-party types) into strings.
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## operator << overload for std::ostream
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This is the standard way of providing string conversions in C++ - and the chances are you may already provide this for your own purposes. If you're not familiar with this idiom it involves writing a free function of the form:
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```cpp
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std::ostream& operator << ( std::ostream& os, T const& value ) {
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os << convertMyTypeToString( value );
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return os;
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}
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```
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(where ```T``` is your type and ```convertMyTypeToString``` is where you'll write whatever code is necessary to make your type printable - it doesn't have to be in another function).
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You should put this function in the same namespace as your type, or the global namespace, and have it declared before including Catch's header.
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## Catch::StringMaker specialisation
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If you don't want to provide an ```operator <<``` overload, or you want to convert your type differently for testing purposes, you can provide a specialization for `Catch::StringMaker<T>`:
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```cpp
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namespace Catch {
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template<>
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struct StringMaker<T> {
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static std::string convert( T const& value ) {
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return convertMyTypeToString( value );
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}
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};
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}
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```
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## Catch::is_range specialisation
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As a fallback, Catch attempts to detect if the type can be iterated
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(`begin(T)` and `end(T)` are valid) and if it can be, it is stringified
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as a range. For certain types this can lead to infinite recursion, so
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it can be disabled by specializing `Catch::is_range` like so:
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```cpp
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namespace Catch {
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template<>
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struct is_range<T> {
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static const bool value = false;
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};
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}
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```
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## Exceptions
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By default all exceptions deriving from `std::exception` will be translated to strings by calling the `what()` method. For exception types that do not derive from `std::exception` - or if `what()` does not return a suitable string - use `CATCH_TRANSLATE_EXCEPTION`. This defines a function that takes your exception type, by reference, and returns a string. It can appear anywhere in the code - it doesn't have to be in the same translation unit. For example:
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```cpp
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CATCH_TRANSLATE_EXCEPTION( MyType const& ex ) {
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return ex.message();
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}
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```
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## Enums
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> Introduced in Catch2 2.8.0.
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Enums that already have a `<<` overload for `std::ostream` will convert to strings as expected.
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If you only need to convert enums to strings for test reporting purposes you can provide a `StringMaker` specialisations as any other type.
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However, as a convenience, Catch provides the `REGISTER_ENUM` helper macro that will generate the `StringMaker` specialiation for you with minimal code.
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Simply provide it the (qualified) enum name, followed by all the enum values, and you're done!
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E.g.
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```cpp
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enum class Fruits { Banana, Apple, Mango };
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CATCH_REGISTER_ENUM( Fruits, Fruits::Banana, Fruits::Apple, Fruits::Mango )
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TEST_CASE() {
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REQUIRE( Fruits::Mango == Fruits::Apple );
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}
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```
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... or if the enum is in a namespace:
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```cpp
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namespace Bikeshed {
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enum class Colours { Red, Green, Blue };
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}
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// Important!: This macro must appear at top level scope - not inside a namespace
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// You can fully qualify the names, or use a using if you prefer
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CATCH_REGISTER_ENUM( Bikeshed::Colours,
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Bikeshed::Colours::Red,
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Bikeshed::Colours::Green,
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Bikeshed::Colours::Blue )
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TEST_CASE() {
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REQUIRE( Bikeshed::Colours::Red == Bikeshed::Colours::Blue );
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}
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```
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## Floating point precision
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> [Introduced](https://github.com/catchorg/Catch2/issues/1614) in Catch2 2.8.0.
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Catch provides a built-in `StringMaker` specialization for both `float`
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and `double`. By default, it uses what we think is a reasonable precision,
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but you can customize it by modifying the `precision` static variable
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inside the `StringMaker` specialization, like so:
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```cpp
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Catch::StringMaker<float>::precision = 15;
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const float testFloat1 = 1.12345678901234567899f;
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const float testFloat2 = 1.12345678991234567899f;
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REQUIRE(testFloat1 == testFloat2);
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```
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This assertion will fail and print out the `testFloat1` and `testFloat2`
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to 15 decimal places.
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---
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[Home](Readme.md#top)
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