C++ loop breaked 'cause the std::find algorithm

Question

I have the next C++ code snippet:

...
static constexpr const char* my_char_array [10] { // Some literals here... } // Member of a class
std::vector<std::string> splitted_input { // Contains C++ strings }
std::vector<std::string> matched_keywords { // The coincident ones will be copied here }

for (int i = 0; i < sizeof(this->my_char_array); i++) {
    std::cout << "Comparing: " << this->my_char*_array[i] << std::endl;
    auto value = std::find(splitted_input.begin(), splitted_input.end(), (std::string) this->my_char_array[i]);
    if ( value != end(splitted_input) ) {
        matched_keywords.push_back(this->keywords[i]);
    }
}

I am iterating over an const char*, looking for a literal that could be inside a vec<string>. When I use the std::find algorithm, the for loop stops on the first iteration (std::cout just outputs the first value on my_char*_array).

Never faced an issue like that. Any idea?

Thanks in advice.

Please post an minimal reproducible example instead of incomplete pseudocode, otherwise it’s not really possible to diagnose bugs. — Konrad Rudolph
– Konrad Rudolph, Commented Oct 10, 2021 at 14:13
my_char*_array? That's not a valid name. Please create a proper minimal reproducible example to show us. — Some programmer dude
– Some programmer dude, Commented Oct 10, 2021 at 14:13
Also, remember that the sizeof operator will return the size of the object in bytes. For an array it will only return the number of elements if the array is an array of char (not char* but plain char). — Some programmer dude
– Some programmer dude, Commented Oct 10, 2021 at 14:15
You probably want sizeof(this->my_char_array)/sizeof(this->my_char_array[0]), to get size of my_char_array array — marcinj
– marcinj, Commented Oct 10, 2021 at 14:16
Its safer to use std::array : static constexpr std::array<const char*, 2> my_char_array = {"dsds", "dddd"}; and my_char_array.size() — marcinj
– marcinj, Commented Oct 10, 2021 at 14:18

marcinj · Accepted Answer · 2021-10-10 15:12:26Z

1

In this line:

for (int i = 0; i < sizeof(this->my_char_array); i++) {

you are using sizeof operator which is returning number of bytes which my_char_array occupies, and this is equal to size of pointer (8 bytes on x64 system) multiplied by number of pointers in your array. So this code is iterating over more elements than actually are in you array which is causing UB (undefined behaviour). The usual solution is to divide by element size:

for (int i = 0; i < sizeof(this->my_char_array)/sizeof(this->my_char_array[0]); i++) {

or even better, replace array with std::array, example:

static constexpr std::array<const char*, 2> my_char_array = {"dsds", "dddd"};

and

for (int i = 0; i < my_char_array.size(); i++) {

and don't forget to #include <array>

answered Oct 10, 2021 at 15:12

marcinj

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2 Comments

Alex Vergara Over a year ago

What a nice answer. Thaks for your time!

marcinj Over a year ago

@Pyzyryab no problem :-)

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