C/C++联合union用作函数参数实例(tcy)

本文主要是介绍C/C++联合union用作函数参数实例(tcy)，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

说明：在此重点介绍C union的具体用法，包括函数参数，指针参数，原理不做说明。
用途：主要用在C语言或和C沟通中；在C++被std::variable替代
区别：union和std::variant
1）共同点：都应用于多个变量类型的自由选择能够保存多个类型列表中任意一个对象
2）不同点：a) 不能从union中派生类；不知当前持有值类型,因此不能有non-trivial成员如std::string(从C++11起可有non-trivial成员，必须有特殊成员函数(复制构造和析构))b) variable当前值类型已知;可有任何指定类型成员;可以派生类c) std::variant用途：用于存储和操作不同类型对象,是一个类型安全的联合体(union).variant实例存储某一类型数据,可指定重复数据类型.  注意：联合只能初始化一个参数；若联合中有字符串时当目前选项是非str按str输出用std::cout报错；非str输出不报错（不是当前类型输出无意义的值）直接使用union存在不知类型，不知是否有值的风险,正确用法是加一个type标志（参考第二部分）

 std::variant实例可参考 ：  C++实现python中的列表list，dict (tcy)

第一部分：直接使用联合作为参数：存在不知类型，不知是否有值的风险

Tpar bar(const Tpar& a, int x) {Tpar b;b.l = a.l + x;return b;
}void bar(const Tpar& a, int x, Tpar* dst) {if (dst)dst->l = a.l + x;
}

Tpar bar(const Tpar* src, int x) {Tpar b;if(src) b.l = src->l + x;return b;
}void bar(const Tpar* src, int x, Tpar* dst) {Tpar b;if (dst) {if (src) dst->l = src->l + x;}
}

void print(Tpar a, const char* mode = "int",const char* no="") {if (mode = "int")std::cout << no << a.l << std::endl;else if (mode = "double")std::cout << no << a.d << std::endl;else if (mode = "str")std::cout << no << a.s << std::endl;else std::cout << "err" << std::endl;
}

 测试：

void test_bar() {Tpar a, b; Tpar *pa = &a;Tpar *pb = &b;a.l = 10;b = bar(a, 100);print(b, "int", "b1=");print(*pb, "int", "b1=");a.l = 20;bar(a, 100,pb);print(b, "int", "b2=");print(*pb, "int", "b2=");pa->l = 30;b = bar(pa, 100);print(b, "int", "b3=");print(*pb, "int", "b3=");pa->l = 40;bar(pa, 100, pb);print(b, "int", "b4=");print(*pb, "int", "b4=");
}

/*
b1=110
b1=110
b2=120
b2=120
b3=130
b3=130
b4=140
b4=140
*/

第二部分：正确用法union+1个type指示：

实例： 

void print(Tcpar a, const char* no = "") {if (a.type == 0) std::cout << no << a.par.l << std::endl;else if (a.type == 0) std::cout << no << a.par.l << std::endl;else if (a.type == 1) std::cout << no << a.par.d << std::endl;else if (a.type == 2) std::cout << no << a.par.s << std::endl;else std::cout << "err" << std::endl;
};void set_data(Tcpar& a, int v) {a.par.l = v; a.type = 0; }
void set_data(Tcpar& a, double v) {a.par.d = v; a.type = 1; }
void set_data(Tcpar& a, char* v) { a.par.s = v;a.type = 2; }

实例1：基本测试
int get_int(const Tcpar& a) {if (a.type != 0)throw std::exception("not int");return a.par.l;
};
double get_double(const Tcpar& a) {if (a.type != 1)throw std::exception("not double");return a.par.d;
};
char* get_str(const Tcpar& a) {if (a.type != 2)throw std::exception("not str");char* p = new char[255];strncpy_s(p, 255, a.par.s, 255-1);return p;
};
void get_str(const Tcpar& a,char* rst,size_t n) {if (a.type != 2)throw std::exception("not str");strncpy_s(rst, n, a.par.s, n-1 - 1);
};

实例2：函数参数
Tcpar foo(const Tcpar& a, int x) {Tcpar b;if (a.type == 0) set_data(b, a.par.l+x); else if (a.type == 1) set_data(b, a.par.d+x); else throw std::exception("param err");return b;
};Tcpar foo(const Tcpar& a, const char* x,size_t total) {Tcpar b;if (a.type == 2) {char* p = new char[total];std::string s = std::string(a.par.s) + x;strncpy_s(p, total, s.c_str(), total -1);b.par.s = p; b.type = 2;}else {throw std::exception("param err");}return b;
};void foo(const Tcpar& a, const char* x,Tcpar* out,size_t total) {if (a.type == 2) {std::string s = a.par.s + std::string(x);strncpy_s(out->par.s, total, s.c_str(), total-1);}else {throw std::exception("param err");}
};

实例3：指针参数
Tcpar foo(const Tcpar* src, int x) {Tcpar b;if (src->type == 0) set_data(b, src->par.l + x);else if (src->type == 1) set_data(b, src->par.d + x);else throw std::exception("param err");return b;
};Tcpar foo(const Tcpar* src, const char* x, size_t total) {Tcpar b;if (src->type == 2) {char* p = new char[total];std::string s = src->par.s + std::string(x);strncpy_s(p, total, s.c_str(), total - 1);b.par.s = p; b.type = 2;}else {throw std::exception("param err");}return b;
};void foo(const Tcpar* src, const char* x, Tcpar* dst, size_t total) {if (src->type == 2) {std::string s = src->par.s + std::string(x);strncpy_s(dst->par.s, total, s.c_str(), total - 1);dst->type = 2;}else {throw std::exception("param err");}
};

测试程序：
void test_union_base() {Tcpar a;a.par.l = 100; a.type = 0;print(a, "int is ");a.par.d = 3.14; a.type = 1;print(a, "double is ");char*p = const_cast<char*>("Tom");a.par.s = p; a.type = 2;print(a, "str is ");std::cout << std::endl;
}void test_set_data() {Tcpar a;char*p = const_cast<char*>("Jim");set_data(a, 200);print(a, "int is ");set_data(a, 6.28);print(a, "double is ");set_data(a, p);print(a, "str is ");std::cout << std::endl;
}
void test_get_data() {Tcpar a;set_data(a, 111);std::cout << get_int(a) << std::endl;set_data(a, 33.3);std::cout << get_double(a) << std::endl;char* p = const_cast<char*> ("China");set_data(a, p);std::cout << get_str(a) << std::endl;char arr[255];get_str(a, arr,255);std::cout << arr << std::endl << std::endl;
}

void test_foo() {Tcpar a;a.par.l = -10; a.type = 0;Tcpar b = foo(a, -100);print(b, "foo(Tcpar a,int x)=");const char* str = "Chongming"; char* p = const_cast<char*>("Shanghai-");a.par.s = p; a.type = 2;b = foo(a, str, 255);print(b, "foo(Tcpar a,const char* x,size_t total)=");p = const_cast<char*>("Nanjing-");a.par.s = p; a.type = 2;Tcpar *pb = &b;foo(a, "Gulou", pb, 255);print(b, "foo(Tcpar a,const char* x,Tcpar* out,size_t total)=");
}void test_foo_ptr() {Tcpar a, b;Tcpar *pa = &a;Tcpar *pb = &b;set_data(a, -99);b = foo(pa, -100);print(b, "b1=");char* p = const_cast<char*> ("Henan-");set_data(a, p);b = foo(pa, "Nanyang", 255);print(b, "b2=");p = const_cast<char*> ("Tianjin-");set_data(a, p);foo(pa, "Tanggu", pb, 255);print(b, "b3=");
} 

void test_union() {std::cout << "start test union base..." << std::endl;test_union_base();std::cout << "start test set union data..." << std::endl;test_set_data();std::cout << "start test get union data..." << std::endl;test_get_data();std::cout << "start test foo..." << std::endl;test_foo();std::cout << "start test foo ptr ..." << std::endl;test_foo_ptr();
}

结果：
/*
start test union base...
int is 100
double is 3.14
str is Tomstart test set union data...
int is 200
double is 6.28
str is Jimstart test get union data...
111
33.3
China
Chinastart test foo...
foo(Tcpar a,int x)=-110
foo(Tcpar a,const char* x,size_t total)=Shanghai-Chongming
foo(Tcpar a,const char* x,Tcpar* out,size_t total)=Nanjing-Gulou
start test foo ptr ...
b1=-199
b2=Henan-Nanyang
b3=Tianjin-Tanggu
*/

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