[英]C++ How to overload the subscript operator and return a function
This question is same as i post.这个问题和我发的一样。 but i not get a good answer and now i try to make it much clear.但我没有得到一个好的答案,现在我试着把它说清楚。 (i hope!!) (我希望!!)
I have a list (vector) that will hold 'key/value' pairs.我有一个包含“键/值”对的列表(向量)。 Example of key/value pair is "Pi = 3.14159" or "UserName = Bob".键/值对的示例是“Pi = 3.14159”或“UserName = Bob”。 The 'Value's can be string, bool, int, float, or doubles. “值”可以是字符串、布尔值、整数、浮点数或双精度数。 I need functions for each data type that i can add to the list, for example.例如,我需要可以添加到列表中的每种数据类型的函数。
Foo f; f["Pi"].SetFloat(3.14159); <--- Add float key/value
Foo f; float myFloat = f["Pi"].GetFloat(); <--- Get float value
or
Foo f; f["UserName"].SetStr("Bob"); <-- Add string key/value
Foo f; const char * name = f["UserName"].GetStr(); < -- Get String.
I have much of the code now.我现在有很多代码。 BUT i do not know how to overload the subscript [].但我不知道如何重载下标 []。
Foo & operator[]( const char * pszKey ){
*this->????
}
The Foo & operator[]( const char * pszKey) should let me find the Key and inside the function should let me add key/value or return a value. Foo & operator[]( const char * pszKey) 应该让我找到密钥,在 function 里面应该让我添加键/值或返回一个值。 I do not know how to add key/value or return the value useing the subscript operator.我不知道如何使用下标运算符添加键/值或返回值。 I hope someone can write a small code to help me understand and help me with is question??我希望有人可以写一个小代码来帮助我理解并帮助我解决问题??
This is some of my code i have now.这是我现在拥有的一些代码。
const char * va( const char * fmt, ... ) { // va = "VarArgs"
// Format a value into a string.
}
void Foo::SetString( const char * Key, const char * ) { // Private
// Add 'key' and 'value' to a vector as a pair.
}
const char * Foo::GetString( const char * key ) { // Private
// Look up 'key' from the vector and return the 'value'.
}
...
const char * Foo::GetStr( const * Key ) {
...
}
void Foo::SetStr( const char * Key, const int Value ) {
SetStr( Key, va("%i", Value ));
}
float Foo::GetFloat( const char * Key ) {
return (float) atof( GetStr( Key ));
}
void Foo::SetFloat( const char * Key ) {
....
}
void Foo::SetInt( .... ) ;
const int * Foo::GetInt(...);
void Foo::SetBool( ... );
const bool Foo::GetBool(...);
The subscript operator should return whatever variant type you are using to store the float, int, string, etc.下标运算符应返回您用于存储浮点数、整数、字符串等的任何变体类型。
And that variant type ( Foo
, is it?) should have assignment and conversion operators:那个变体类型( Foo
,是吗?)应该有赋值和转换运算符:
struct my_variant_type
{
...
operator int () const { return GetInt(); }
operator float () const { return GetFloat(); }
operator string() const { return GetStr(); }
etc
my_variant_type & operator = ( int n ) { SetInt( n ); return *this; }
my_variant_type & operator = ( float f ) { SetFloat( f ); return *this; }
my_variant_type & operator = ( const std::string & s ) { SetStr( s ); return *this; }
etc
};
Now you can say things like:现在你可以这样说:
float pi = my_collection["pi"];
my_collection["name"] = "Harry Potter";
I do not understand exactly why not just use std::map<std::string, std::variant<...>>
and use operator=
with variant constructor for assignment and std::get<>
for getting value of proper type.我不明白为什么不只使用std::map<std::string, std::variant<...>>
并使用operator=
和变体构造函数进行赋值,使用std::get<>
获取正确的值类型。 Anyway, here's a wrapper around with requested interface that you proposed:无论如何,这是一个包装器,带有您建议的请求接口:
#include <variant>
#include <string>
#include <map>
struct Foo {
using Value = std::variant<
std::string,
bool,
int,
float,
double>;
std::map<std::string, Value> data;
struct YourInterface {
Value& v;
float GetFloat() {
return std::get<float>(v);
}
void SetFloat(float f) {
v = Value(f);
}
void SetStr(std::string n) {
v = Value(n);
}
std::string GetStr() {
return std::get<std::string>(v);
}
// etc.
};
YourInterface operator[](const char *pszKey) {
return FooInterface(data[pszKey]);
}
};
int main() {
Foo f;
f["UserName"].SetStr("Bob");
float myFloat = f["Pi"].GetFloat();
f["UserName"].SetStr("Bob");
std::string name = f["UserName"].GetStr();
}
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