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Diffstat (limited to 'doc/TemplateKeyword.dox')
| -rw-r--r-- | doc/TemplateKeyword.dox | 17 |
1 files changed, 9 insertions, 8 deletions
diff --git a/doc/TemplateKeyword.dox b/doc/TemplateKeyword.dox index f4e4f237e..b84cfdae9 100644 --- a/doc/TemplateKeyword.dox +++ b/doc/TemplateKeyword.dox @@ -5,7 +5,8 @@ namespace Eigen { There are two uses for the \c template and \c typename keywords in C++. One of them is fairly well known amongst programmers: to define templates. The other use is more obscure: to specify that an expression refers to a template function or a type. This regularly trips up programmers that use the %Eigen library, often -leading to error messages from the compiler that are difficult to understand. +leading to error messages from the compiler that are difficult to understand, such as "expected expression" or +"no match for operator<". \eigenAutoToc @@ -72,23 +73,23 @@ for operator<". The reason that the \c template keyword is necessary in the last example has to do with the rules for how templates are supposed to be compiled in C++. The compiler has to check the code for correct syntax at the point where the template is defined, without knowing the actual value of the template arguments (\c Derived1 -and \c Derived2 in the example). That means that the compiler cannot know that <tt>dst.triangularPart</tt> is +and \c Derived2 in the example). That means that the compiler cannot know that <tt>dst.triangularView</tt> is a member template and that the following < symbol is part of the delimiter for the template -parameter. Another possibility would be that <tt>dst.triangularPart</tt> is a member variable with the < +parameter. Another possibility would be that <tt>dst.triangularView</tt> is a member variable with the < symbol refering to the <tt>operator<()</tt> function. In fact, the compiler should choose the second -possibility, according to the standard. If <tt>dst.triangularPart</tt> is a member template (as in our case), +possibility, according to the standard. If <tt>dst.triangularView</tt> is a member template (as in our case), the programmer should specify this explicitly with the \c template keyword and write <tt>dst.template -triangularPart</tt>. +triangularView</tt>. The precise rules are rather complicated, but ignoring some subtleties we can summarize them as follows: - A <em>dependent name</em> is name that depends (directly or indirectly) on a template parameter. In the example, \c dst is a dependent name because it is of type <tt>MatrixBase<Derived1></tt> which depends on the template parameter \c Derived1. -- If the code contains either one of the contructions <tt>xxx.yyy</tt> or <tt>xxx->yyy</tt> and \c xxx is a +- If the code contains either one of the constructs <tt>xxx.yyy</tt> or <tt>xxx->yyy</tt> and \c xxx is a dependent name and \c yyy refers to a member template, then the \c template keyword must be used before \c yyy, leading to <tt>xxx.template yyy</tt> or <tt>xxx->template yyy</tt>. -- If the code contains the contruction <tt>xxx::yyy</tt> and \c xxx is a dependent name and \c yyy refers to a - member typedef, then the \c typename keyword must be used before the whole construction, leading to +- If the code contains the construct <tt>xxx::yyy</tt> and \c xxx is a dependent name and \c yyy refers to a + member typedef, then the \c typename keyword must be used before the whole construct, leading to <tt>typename xxx::yyy</tt>. As an example where the \c typename keyword is required, consider the following code in \ref TutorialSparse |