{"id":33718,"date":"2023-02-12T21:51:51","date_gmt":"2023-02-12T16:21:51","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-changing-vector-in-function-by-pointer\/"},"modified":"2023-02-12T21:51:51","modified_gmt":"2023-02-12T16:21:51","slug":"solved-changing-vector-in-function-by-pointer","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-changing-vector-in-function-by-pointer\/","title":{"rendered":"[Solved] Changing vector in function by pointer"},"content":{"rendered":"

[ad_1]
\n<\/p>\n

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<\/div>\n
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The function does not make sense.<\/p>\n

For starters it is unclear why you are using a pointer to the vector instead of a reference to the vector.<\/p>\n

Nevertheles, this declaration<\/p>\n

vector<unsigned char> vec(5);\n<\/code><\/pre>\n
does not reseeve a memory for the vector. It initializes the vector with 5 zero-characters which will be appended with other characters in the function.<\/p>\n
The expression<\/p>\n
&(*result).back()\n<\/code><\/pre>\n
returns reference to the last element of the vector.  And this call<\/p>\n
memcpy(&(*result).back(), elem, elem_size);\n<\/code><\/pre>\n
will try to overwrite memory that does not belong to the vector. As result the function has undefined behaviour.<\/p>\n
Youy can imagine the situation the following way<\/p>\n
   vector\n|0|0|0|0|0|\n        |a|b|c|d|\n         string\n<\/code><\/pre>\n
so as it is seen there is an attempt to copy characters ‘b’, ‘c’, and ‘d’ to the memory that does not belong to the vector.<\/p>\n
You should use methods of the class template std::vector<\/code> to append new elements.<\/p>\n
The function can look the following way as it is shown in the demonstrative program (provided that you want to use a pointer to vector).<\/p>\n
#include <iostream>\n#include <vector>\n#include <string>\n\nvoid addElem( std::vector<unsigned char> *result, const std::string &str_to_add ) \n{\n    result->reserve( result->size() + str_to_add.size() );\n    result->insert( result->end(), str_to_add.begin(), str_to_add.end() );\n}   \n\nint main() \n{\n    std::vector<unsigned char> vec;\n    std::vector<unsigned char> *vec_ptr = &vec;\n\n    addElem( vec_ptr, \"abcd\" );\n\n    for ( auto c : *vec_ptr ) std::cout << c;\n    std::cout << std::endl;\n\n    return 0;\n}\n<\/code><\/pre>\n
Its output is<\/p>\n
abcd\n<\/code><\/pre>\n<\/p><\/div>\n
<\/div>\n<\/div>\n
            1<\/span> <\/p><\/div>\n<\/div>\n
[ad_2]<\/p>\n
solved Changing vector in function by pointer <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] The function does not make sense. For starters it is unclear why you are using a pointer to the vector instead of a reference to the vector. Nevertheles, this declaration vector<unsigned char> vec(5); does not reseeve a memory for the vector. It initializes the vector with 5 zero-characters which will be appended with other … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[1537,324,712,362,806],"class_list":["post-33718","post","type-post","status-publish","format-standard","hentry","category-solved","tag-append","tag-c","tag-pointers","tag-string","tag-vector"],"yoast_head":"\n[Solved] Changing vector in function by pointer - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-changing-vector-in-function-by-pointer\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Changing vector in function by pointer - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] The function does not make sense. For starters it is unclear why you are using a pointer to the vector instead of a reference to the vector. Nevertheles, this declaration vector<unsigned char> vec(5); does not reseeve a memory for the vector. It initializes the vector with 5 zero-characters which will be appended with other ... 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For starters it is unclear why you are using a pointer to the vector instead of a reference to the vector. Nevertheles, this declaration vector<unsigned char> vec(5); does not reseeve a memory for the vector. It initializes the vector with 5 zero-characters which will be appended with other ... 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