Leeios Leeios - 2 months ago 21
C++ Question

C++ Template-like behavior in Go

For example I have two different structs

Foo
and
Bar


struct Foo {...}
struct Bar {...}


and a lot of functions and other types built on them.

C++ flavor:

template<typename T>
struct Identified {
T model;
std::string id;
};

template<typename T>
Identified<T> GetIdentifiedModel(std::string id) {
Identified<T> result;
T.id = id;
T.model.set(getSomeData(id)); // Common method for T
return result;
}


How to implement these examples in Go?

For common methods, interfaces do the job but I don't see how to retrieve a specific type from an interface to declare it, return it or anything else and I can't handle copy/paste code anymore :)

Thanks !

Edit @Amd: https://ideone.com/rqpsQb

#include <string>
#include <iostream>

struct Foo {
char c;
void set(std::string s) {c = s[0];}; // We don't really care here
};
struct Bar {
int n;
void set(std::string s) {n = s.size();}; // We don't really care here
};

template<typename T>
struct Identified {
T model;
std::string id;
};

template<typename T>
Identified<T> GetIdentifiedModel(std::string id) {
Identified<T> result;
result.id = id;
// Obviously shouldn't be ID but for the example
result.model.set(id); // Common method for T
return result;
}

void assert(bool b) {
if (b) std::cout << "OK" << std::endl;
else std::cout << "There is a problem !" << std::endl;
};

int main() {
auto fooWithID = GetIdentifiedModel<Foo>("foo id");
auto barWithID = GetIdentifiedModel<Bar>("bar");
assert (fooWithID.model.c == 'f');
assert (barWithID.model.n == 3);
return (0);
}

Amd Amd
Answer

1- You may use

fooWithID := GetIdentifiedModel("foo id", &Foo{})

like this working sample ( try on The Go Playground):

package main

import "fmt"

type Foo struct {
    c byte
}

func (t *Foo) set(s string) { t.c = s[0] }

type Bar struct {
    n int
}

func (t *Bar) set(s string) { t.n = len(s) }

type Identified struct {
    model T
    id    string
}

func GetIdentifiedModel(id string, t T) *Identified {
    result := &Identified{model: t}
    result.id = id
    result.model.set(id)
    return result
}

func assert(b bool) {
    if b {
        fmt.Println("OK")
    } else {
        fmt.Println("There is a problem !")
    }
}

func main() {
    fooWithID := GetIdentifiedModel("foo id", &Foo{})
    barWithID := GetIdentifiedModel("bar", &Bar{})

    assert(fooWithID.model.(*Foo).c == 'f')
    assert(barWithID.model.(*Bar).n == 3)
}

type T interface {
    set(string)
}

output:

OK
OK

2- You may use (this is nice to read: Identified model Foo):

fooWithID := GetIdentifiedModel("foo id", &Identified{model: &Foo{}})

like this working sample ( try on The Go Playground):

package main

import "fmt"

type Foo struct {
    c byte
}

func (t *Foo) set(s string) { t.c = s[0] }

type Bar struct {
    n int
}

func (t *Bar) set(s string) { t.n = len(s) }

type Identified struct {
    model T
    id    string
}

func GetIdentifiedModel(id string, result *Identified) *Identified {
    result.id = id
    result.model.set(id)
    return result
}

func assert(b bool) {
    if b {
        fmt.Println("OK")
    } else {
        fmt.Println("There is a problem !")
    }
}

func main() {
    fooWithID := GetIdentifiedModel("foo id", &Identified{model: &Foo{}})
    barWithID := GetIdentifiedModel("bar", &Identified{model: &Bar{}})    
    assert(fooWithID.model.(*Foo).c == 'f')
    assert(barWithID.model.(*Bar).n == 3)
}

type T interface {
    set(string)
}

output:

OK
OK

See also: One method to handle all the struct types that embed one common struct (json marshalling)

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