Default template parameter redeclaration

[temp.param]/12 Clearly states :

A template-parameter shall not be given default arguments by two different declarations in the same scope.

now, both clang and g++ generates errors for following code:

template<class T = int> class X;
template<class T = int> class X {};

msvc on the other hand (even with no extensions enabled) compiles it fine, I had to change second default parameter from int to some other type to see error. So its not that really bad, but still ‘shall not’ means ‘is not allowed’ as per ISO/IEC Directives. msvc on the other hand gives error in similar case but with redeclaration of function default parameter values.

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String literal as non type template parameter

Unfortunately it is not possible to have template with string literal as a non type parameter. Until C++11 reason was that string literals had internal linkage, while non type template parameters allowed only external linkage pointers. It makes sense because templ_class should have the same typeid in all translation units. It kind of could work with string pooling enabled – which is availabe for both gcc and msvc. After C++11 the reason is more of that string literals does not match any of the allowed entities as template parameter.

You can use character array initialized with string literal as non type parameter. It compiles both with g++ and msvc:

#include <typeinfo>

template<const char* STR>
class MyStr {};

extern const char my_string[]; const char my_string[] = "alfabet";
extern const char my_string2[]; const char my_string2[] = "alfabet";

// or use constexpr
//constexpr const char my_string[] = "alfabet";
//constexpr const char my_string2[] = "alfabet";

extern const char* my_string_pc;
const char* my_string_pc = "alfabet";

int main(){
   
    // This fails with:
    //  main.cpp:30:31: error: 'my_string_pc' is not a valid template argument because 'my_string_pc' is a variable, not the address of a variable
    //    typedef MyStr<my_string_pc> str_type3;
    typedef MyStr<my_string_pc> str_type3;     
   
    // Below compiles, but gives different types
    typedef MyStr<my_string> str_type1;   
    typedef MyStr<my_string2> str_type2; 
   
    // This fails
    static_assert(std::is_same<str_type1, str_type2>::value, "");
   
    return 0;
}

EDIT:
After writing it I have found in standard ([temp.arg.nontype]) the actual reasoning for all above:

[ Note: A string literal (2.14.5) does not satisfy the requirements of any of these categories and thus is not
an acceptable template-argument. [ Example:

template<class T, const char* p> class X {
/ ... /
};
X<int, "Studebaker"> x1; // error: string literal as template-argument
const char p[] = "Vivisectionist";
X<int,p> x2; // OK

—end example ] —end note ]

refs:
http://stackoverflow.com/questions/5547852/string-literals-not-allowed-as-non-type-template-parameters

Linkage of templates

I would like to share observation of another non standard behavior found in Visual Studio compiler. According to C++ Templates Complete Guide: “class templates cannot share a name with a different kind of entity”, below code:

int X;
template
class X {    
};

gives following error with g++ 4.9:


main.cpp:8:7: error: 'template struct X' redeclared as different kind of symbol
class X {
^
main.cpp:6:5: note: previous declaration 'int X'
int X;


but with VS2013 compiles just fine.

Be carefull with subnormal floats

Subnormal floating point numbers are values that are very small (.0f to ~.1f), operations on them are specially treated by CPUs. Below is a timing of simple sumation of floating point values, you can see that once subnormal values are used, performance instantly drops 10 or more times.

http://coliru.stacked-crooked.com/a/6ffe763ab4b5ada0

// Example code for testing subnormal float efficency
float f = 2.0f;
std::chrono::steady_clock::duration prevdiff;
for (int k = 0; k < 10000; ++k) {
  float sum = .0f;		
  auto start = std::chrono::steady_clock::now();

  for (int n = 0; n < 1e7; ++n) {
    sum += f * 0.1f;  // interesting, but simple summation show no performance degradation
  }

  auto end = std::chrono::steady_clock::now();
  auto diff = end - start;
  std::cout << k << ", f=" << f << ", isnormal=" << std::isnormal(f)
    << ", elapsed=" << std::chrono::duration_cast<std::chrono::milliseconds>(diff).count() << "ms" <<
    ", ratio=" << (prevdiff.count() == 0 ? 0 : (diff.count() / prevdiff.count())) << std::endl;
    prevdiff = diff;

   if (f < std::numeric_limits<float>::min())
     break;

    f /= 2.0f;
}

here is the interesting part from the output:

123, f=1.88079e-37, isnormal=1, elapsed=69ms, ratio=1
124, f=9.40395e-38, isnormal=1, elapsed=799ms, ratio=11 <-------- here performance drop
125, f=4.70198e-38, isnormal=1, elapsed=800ms, ratio=1
126, f=2.35099e-38, isnormal=1, elapsed=800ms, ratio=0
127, f=1.17549e-38, isnormal=1, elapsed=796ms, ratio=0
128, f=5.87747e-39, isnormal=0, elapsed=985ms, ratio=1

whats interesting is that this drop happens before std::isnormal returns true. You can use _MM_SET_FLUSH_ZERO_MODE(_MM_FLUSH_ZERO_ON); to make CPU not do computations using subnormal values.

This post is based on following SO : http://stackoverflow.com/questions/9314534/why-does-changing-0-1f-to-0-slow-down-performance-by-10x.

Dependent base class name lookup (c++ template complete guide)

If you were reading c++ template complete guide : “5.2 using this->” you probably tried to test following example:

template<typename T>
class Base {
public:
  void exit() {}
};

template<typename T>
class Derived : public Base < T > {
public:
	void callMe() {
	  exit();
	}
};

if you done it under Visual Studio, you have probably realized that it does compile with no errors or warnings, so what is going on here. Your next step should be to test it with gcc or clang compiler, you would see errors now.

The problem is that VS does not implement two phase name lookup. I have found that in VS2013 diabling language extensions with /Za actually causes above code to fail to compile.

some usefull links below
http://blog.llvm.org/2009/12/dreaded-two-phase-name-lookup.html
http://stackoverflow.com/questions/11405/gcc-problem-using-a-member-of-a-base-class-that-depends-on-a-template-argument
http://clang.llvm.org/compatibility.html#dep_lookup_bases