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  1. #include <cassert>
  2. #include <iostream>
  3.  
  4. namespace expr
  5. {
  6. 	typedef unsigned int uint;
  7.  
  8. 	// globals to count constructor and destructor calls
  9. 	// of a vec
  10. 	uint ctor_calls = 0;
  11. 	uint dtor_calls = 0;
  12. 	// print the numbers
  13. 	void printctor()
  14. 	{
  15. 		std::cout << "ctor/dtor = " << ctor_calls 
  16.                 << "/" << dtor_calls << "\n";
  17. 	}
  18.  
  19. 	// an example vec container of three successive floats
  20. 	class vec
  21. 	{
  22. 		public:
  23.  
  24. 		float v[3];
  25.  
  26. 		// note: the {...} is c++0x initializer syntax
  27. 		vec () : v({0,0,0}) { ++ctor_calls; }
  28. 		vec (const vec& o) : v({o.v[0], o.v[1], o.v[2]}) { ++ctor_calls; }
  29. 		vec (float x,float y,float z) : v({x,y,z}) { ++ctor_calls; }
  30.  
  31. 		~vec() { ++dtor_calls; }
  32.  
  33. 		// return indexed value
  34. 		float operator[](uint index) const { return v[index]; }
  35. 		// return indexed reference
  36. 		float& operator[](uint index) { return v[index]; }
  37.  
  38. 		void print() { std::cout << "<"<<v[0]<<","<<v[1]<<","<<v[2]<<">\n"; }
  39.  
  40. 		// assignment to an expression
  41.                 // E must have operator[](uint)
  42. 		template <class E>
  43. 		vec& operator= (const E& x)
  44. 		{
  45. 			for (uint i=0; i!=3; ++i) (*this)[i] = x[i];
  46. 			return *this;
  47. 		}
  48. 	};
  49.  
  50.  
  51. 	// basic catch-all expression node
  52. 	// L and R must provide operator[](uint)
  53. 	// O must provide static function float eval(float,float)
  54. 	template <class L, class O, class R>
  55. 	struct expression
  56. 	{
  57. 		expression(const L& l, const R& r)
  58. 			:	l(l), r(r) { }
  59.  
  60. 		float operator[](const uint index) const
  61. 		{
  62. 			return O::eval(l[index], r[index]);
  63. 		}
  64.  
  65. 		const L& l;
  66. 		const R& r;
  67. 	};
  68.  
  69. 	// wraps a reference to float into an operator[](uint) entity
  70. 	class scalar
  71. 	{
  72. 		public:
  73.  
  74. 		scalar(const float& t) : t(t) { }
  75.  
  76. 		// act like an endless vector of ts
  77. 		float operator[](uint) const { return t; }
  78.  
  79. 		const float& t;
  80. 	};
  81.  
  82. 	// an operation function object
  83. 	struct plus
  84. 	{
  85. 		static float eval(const float a, const float b) { return a + b; }
  86. 	};
  87.  
  88. 	// anything + anything
  89. 	template <class L, class R>
  90. 	expression<L,plus,R> operator+(const L& l, const R& r)
  91. 	{
  92. 		return expression<L,plus,R>(l, r);
  93. 	}
  94.  
  95. 	// anything + scalar
  96. 	template <class L>
  97. 	expression<L,plus,scalar> operator+(const L& l, const float& r)
  98. 	{
  99. 		return expression<L,plus,scalar>(l, r);
  100. 	}
  101. }
  102.  
  103. void do_some()
  104. {
  105. 	using namespace expr;
  106.  
  107. 	vec a(1,2,3), b(2,3,4), c(3,4,5);
  108. 	a.print(); b.print(); c.print();
  109.  
  110. 	// works
  111. 	a = b + c;
  112. 	a.print();
  113. 	assert( a.v[0] == 5 && a.v[1] == 7 && a.v[2] == 9 );
  114.  
  115. 	// does not work -> segfault
  116. 	a = b + 1.f;
  117. 	a.print();
  118. 	assert( a.v[0] == 3 && a.v[1] == 4 && a.v[2] == 5 );
  119. }
  120.  
  121. int main()
  122. {
  123. 	do_some();
  124.  
  125. 	// check ctor calls
  126. 	expr::printctor();
  127.  
  128.     return 0;
  129. }
Success #stdin #stdout 0s 2884KB
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stdin
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Standard input is empty
stdout
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<1,2,3>
<2,3,4>
<3,4,5>
<5,7,9>
<3,4,5>
ctor/dtor = 3/3
https://ideone.com/nS1L0
language:
C++14 (gcc 8.3)
created:
13 years ago
visibility:
public

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