c++ initialize char array member of a class with a string

Question

i have a class which has several members of char arrays, and i want to initialize this class with an array of strings which are the values of the char arrays.

class Product {
public:
    char productId[20];
    char productName[50];
    char price[9];
    char stock[9];

    Product(vector<string> v) : productId(v[0]), productName(v[1]), price(v[2]), stock(v[3]) {  }
};

with this code i get an error that say no suitable conversion function from "str::string" to "char[20]" exist

Have you thought about what should happen if a std::string is too long to fit into its fixed character array? — Drew Dormann
– Drew Dormann, Commented Dec 9, 2021 at 20:40
At least pass the vector by reference so you don't invoke unnecessary allocations and additional copies. If you can't guarantee safe string lengths at runtime to avoid overflowing these, consider strncpy_s. — paddy
– paddy, Commented Dec 9, 2021 at 20:41
thank you all for your answers, first i have no priority with memory safety or stuff like that, i'm a new learner of c++ and i'm stuck with this. Second i've tried with strcpy but i got an error like this: a value of type "char *" cannot be used to initialize an entity of type "char [20] — InspectorGadget
– InspectorGadget, Commented Dec 9, 2021 at 20:45

Ton van den Heuvel · Accepted Answer · 2021-12-09 23:03:35Z

1

The code at the bottom will work. But is this a good idea? Probably not. You are better of just storing std::string in your Product type directly:

#include <cassert>
#include <iostream>
#include <vector>

#include <string.h>

class Product {
public:
  std::string productId;
  ...

  Product(std::vector<std::string> v) : productId{std::move(v[0])} {}
};

There is a problem with this code though; where do you check the vector has all required elements? Better to make an interface that specifies the four strings a Product is made up of separately:

class Product {
public:
  std::string productId;
  ...

  Product(std::string pid, ...) : productId{std::move(pid)}, ... {}
};

But in case you insist on a C/C++ amalgamation;

#include <cassert>
#include <vector>

#include <string.h> // strcpy

class Product {
public:
  char productId[20];
  char productName[50];
  char price[9];
  char stock[9];

  Product(const std::vector<std::string> &v) {
    assert(!v.empty()); // really; v.size() == 4
    ::strncpy(productId, v[0].c_str(), 19);
    productId[19] = '\0';
    // ...etc.
  }
};

edited Dec 9, 2021 at 23:03

answered Dec 9, 2021 at 20:55

Ton van den Heuvel

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2 Comments

Remy Lebeau Over a year ago

In your last example, you should use (std::)strncpy() instead to avoid potential buffer overflows, and should ensure the data is actually null-terminated, eg: strncpy(productId, v[0].c_str(), 19); productId[19] = '\0';

Ton van den Heuvel Over a year ago

@RemyLebeau, yes, I've updated the answer.

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