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std::max与max宏 混淆解决办法(转)

时间:2016-09-22 10:03:27      阅读:186      评论:0      收藏:0      [点我收藏+]

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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.

std::max与max宏 混淆解决办法(转)

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原文地址:http://www.cnblogs.com/timesdaughter/p/5894930.html

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