How to initialize an array of structs that demand arguments in C++

Question

I'm trying to initialize an array of structs, but the structs demand arguments upon initialization, like this:

#include <iostream>
using namespace std;

struct MyStruct{
    string par1;
    double par2;
    MyStruct(string var1, double var2){
        par1=var1;
        par2=var2;
    }
};

int main(){
    MyStruct test("apples", 17.5);
    MyStruct MyArray[3];
return 0;
}

the structure 'test' initializes without problem, but when MyArray tries to initialize, it gives me

'no matching function for call to MyStruct::MyStruct()'

and also

'candidate expects 2 arguments, 0 provided'

I assume this is because I have to initialize it using some arguments for par1 and par2, but I don't know how to, or if it's even possible. I would appreciate any help, thank you.

Vector allows you to construct one object and copy-construct it to make a bunch of them. If you wish all objects to be initialized differently, you can use a loop to construct one by one and push_back. — user3528438
– user3528438, Commented Apr 10, 2015 at 15:05

Mike Seymour · Accepted Answer · 2015-04-10 15:00:24Z

4

Provide an initialiser for each array element:

MyStruct MyArray[3] = {
    MyStruct("oranges", 42.6),
    MyStruct("bananas", 63.2),
    MyStruct("pomegranates", 3.2)
};

Since C++11, this can be reduced slightly:

MyStruct MyArray[3] {
    {"oranges", 42.6},
    {"bananas", 63.2},
    {"pomegranates", 3.2}
};

answered Apr 10, 2015 at 15:00

Mike Seymour

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4 Comments

Simeon Over a year ago

Ah, so I was just missing the syntax. How do I initialize the whole array to a default value such as MyStruct("nil", 0) and add arguments later? Whenever I try that I get "error: could not convert '<brace-enclosed initializer list>()' from '<brace-enclosed initializer list>' to 'MyStruct'"

Mike Seymour Over a year ago

@Simeon: You don't. You either initialise them all individually, or provide a default constructor to value-initialise ones with missing initialisers. Or use std::vector rather than an array.

Simeon Over a year ago

How do I provide a constructor that only initializes the ones with missing arguments/initialisers?

Mike Seymour Over a year ago

@Simeon: By declaring and defining a default constructor in the usual manner. You can't constrain it only to be used in an array with missing initialisers, so if you don't want it to be default-constructible in general, you'll have to use one of the other suggestions.

user3528438 · Accepted Answer · 2015-04-10 15:15:13Z

1

int main(){
    std::vector<MyStruct> MyArray(3, MyStruct("apples", 17.5));
return 0;
}

This method has some limitations: 1)elements in the vector are the same, 2)class has to be copy-constructable.

answered Apr 10, 2015 at 15:15

user3528438

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2 Comments

Simeon Over a year ago

This is exactly what I wanted, since my array has to be initialized very long and then modified afterwards. Is there a way to do it without using the vector library?

user3528438 Over a year ago

@Simeon, unless you re-write MyStruct::MyStruct(void). But I really don't think there is a reason to avoid std::vector, for you can do &MyArray[0] to "down grade" it to a raw pointer/array.

nils · Accepted Answer · 2015-04-10 15:00:00Z

0

Is there any reason against the use of std::vector?

#include <vector>

std::vector< MyStruct > mystructs;

mystructs.push_back( MyStruct( "apples",   17.5 ) );
mystructs.push_back( MyStruct( "bananas",   7.5 ) );
mystructs.push_back( MyStruct( "cherries", 17.5 ) );

answered Apr 10, 2015 at 15:00

nils

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5 Comments

Mike Seymour Over a year ago

Not my downvote, but there's no good reason to add unnecessary memory allocations when all that's needed is a simple automatic array.

David G Over a year ago

@MikeSeymour You don't always know at initialization time what the value of the objects will be. You will ultimately need dynamic allocation for non default-constructible objects which std::vector facilitates.

user3528438 Over a year ago

Vector can copy-construct one object into an array/vector, hence easier to use if number of objects is large and objects don't need to be initialized with different parameters.

nils Over a year ago

@MikeSeymour, 0x499602D2, user3528438, thanks for your comments. I agree with all three of them. It probably depends on the specific case which approach is more suitable.

Kerrek SB Over a year ago

I'd say this ignores the OP's question and talks about something else, and the OP sounded neither confused nor insane.

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