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            (搬運工)std::min and std::max

            Posted on 2012-01-06 16:40 點點滴滴 閱讀(1507) 評論(0)  編輯 收藏 引用 所屬分類: 02 編程語言
            Today I typed the following:
            int t = (std::max)(timeout, lagtime);

            Why did I put parentheses around std::max? Because windows.h defines (among other things) a max and a min macro. If you include windows.h the above code will not compile. For example the following:
            #include "windows.h"
            #include <algorithm>

            void foo() {
            int i = 5;
            int j = 7;
            int x = std::max(i,j);
            }

            Will produce the following error with Visual Studio C++ 2005:
            1>test.cpp(7) : error C2589: '(' : illegal token on right side of '::'
            1>test.cpp(7) : error C2143: syntax error : missing ';' before '::'

            There are a number of ways to work around windows.h defining these two macros.

            • Use alternative names defined in windows.h.
              int x = _cpp_max(i,j);
              int y = _cpp_min(i,j);
              This is not portable; only works on Windows.

            • Define NOMINMAX before including windows.h. This might break existing code that assumes NOMINMAX is not defined.

            • Don't use std::min and std::max. Instead use the tertiary operator like so:
              int x = i > j ? i : j; // max(i,j)
              int y = i < j ? i : j; // min(i,j)
              This is portable but not as readable and more error prone.

            • Use using statements to make the code portable:
              using std::min;
              using std::max;
              int x = max(i,j);
              int y = min(i,j);
              This works but requires two more lines of code. You could also just use 'using namespace std;' but that might pull in more than you want.

            • Use std::min<int> and std::max<int>
              int x = std::max<int>(i,j);
              int y = std::min<int>(i,j);
              This requires you to specify the type. However in some cases this actually helps. For example:
              int i = 5;
              unsigned int j = 7;
              int x = (std::max)(i,j);
              int y = (std::min)(i,j);
              Note the 'unsigned'. Generates the following errors:
              1>test.cpp(7) : error C2780: 'const _Ty &std::max(const _Ty &,const _Ty &,_Pr)' : 
              expects 3 arguments - 2 provided
              1> c:\program files\microsoft visual studio 8\vc\include\xutility(3190) :
              see declaration of 'std::max'
              1>test.cpp(7) : error C2782: 'const _Ty &std::max(const _Ty &,const _Ty &)' :
              template parameter '_Ty' is ambiguous
              1> c:\program files\microsoft visual studio 8\vc\include\xutility(3182) :
              see declaration of 'std::max'
              1> could be 'unsigned int'
              1> or 'int'
              By explicitly specifying type via <int> you remove the ambiguity.

            • Use (std::min) and (std::max)
              int i = 5;
              int j = 7;
              int x = (std::max)(i,j);
              int y = (std::min)(i,j);
              This works (as does the std::max<int>) because the C++ preprocessor requires '(' as the next preprocessing token following the macro name to preform the macro expansion.
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