std::max 错误
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.