將char* 賦值給std::string的一些陷阱
阿新 • • 發佈：2019-01-01
這段時間，總是要使用char或者char* 賦值給std::string，踩了不少坑。於是寫了個測試程式碼，如果你不想看我的程式碼，可以跳到下面直接看總結：
輸出結果是：#include <string> #include <iostream> using namespace std; int main(int argc, char* argv[]) { string str1; cout << "str1:" << str1 << " length: " << str1.length() << " size: " << str1.size() << " capacity: " << str1.capacity() << endl; string str2("hello"); cout << "str2:" << str2 << " length: " << str2.length() << " size: " << str2.size() << " capacity: " << str2.capacity() << endl; string str3(4, 'A'); cout << "str3:" << str3 << " length: " << str3.length() << " size: " << str3.size() << " capacity: " << str3.capacity() << endl; string str4(str2); cout << "str4:" << str4 << " length: " << str4.length() << " size: " << str4.size() << " capacity: " << str4.capacity() << endl; cout << "-----------------------------------------------" << endl; char sz1[10] = { 0 }; sz1[0] = 'a'; sz1[1] = 'b'; sz1[2] = 'c'; char sz2[10] = { 0 }; sz2[0] = 'a'; sz2[3] = 'b'; sz2[4] = 'c'; string str5(sz1); string str6(sz1, 2); string str71(sz1, 4); string str72(sz1, 10); string str73(sz1, 30); cout << "str5:" << str5 << " length: " << str5.length() << " size: " << str5.size() << " capacity: " << str5.capacity() << endl; cout << "str6:" << str6 <<" length: " << str6.length() << " size: " << str6.size() << " capacity: " << str6.capacity() << endl; cout << "str71:" << str71 << " length: " << str71.length() << " size: " << str71.size() << " capacity: " << str71.capacity() << endl; cout << "str72:" << str72 << " length: " << str72.length() << " size: " << str72.size() << " capacity: " << str72.capacity() << endl; cout << "str73:" << str73 << " length: " << str73.length() << " size: " << str73.size() << " capacity: " << str73.capacity() << endl; cout << "-----------------------------------------------" << endl; string str8(sz2); string str9(sz2, 1); string str10(sz2, 2); string str11(sz2, 4); string str12(sz2, 10); cout << "str8:" << str8 <<" length: " << str8.length() << " size: " << str8.size() << " capacity: " << str8.capacity() << endl; cout << "str9:" << str9 << " length: " << str9.length() << " size: " << str9.size() << " capacity: " << str9.capacity() << endl; cout << "str10:" << str10 << " length: " << str10.length() << " size: " << str10.size() << " capacity: " << str10.capacity() << endl; cout << "str11:" << str11 << " length: " << str11.length() << " size: " << str11.size() << " capacity: " << str11.capacity() << endl; cout << "str12:" << str12 << " length: " << str12.length() << " size: " << str12.size() << " capacity: " << str12.capacity() << endl; cout << "-----------------------------------------------" << endl; string str13 = sz1; string str14 = sz2; cout << "str13:" << str13 << " length: " << str13.length() << " size: " << str13.size() << " capacity: " << str13.capacity() << endl; cout << "str14:" << str14 << " length: " << str14.length() << " size: " << str14.size() << " capacity: " << str14.capacity() << endl; cout << "-----------------------------------------------" << endl; string str15; str15.append(sz1); string str16; str16.append(sz1, 2); cout << "str15:" << str15 << " length: " << str15.length() << " size: " << str15.size() << " capacity: " << str15.capacity() << endl; cout << "str16:" << str16 << " length: " << str16.length() << " size: " << str16.size() << " capacity: " << str16.capacity() << endl; cout << "-----------------------------------------------" << endl; string str17; str17.append(sz2); string str18; str18.append(sz2, 3); string str19; str19.append(sz2, 10); cout << "str17:" << str17 << " length: " << str17.length() << " size: " << str17.size() << " capacity: " << str17.capacity() << endl; cout << "str18:" << str18 << " length: " << str18.length() << " size: " << str18.size() << " capacity: " << str18.capacity() << endl; cout << "str19:" << str19 << " length: " << str19.length() << " size: " << str19.size() << " capacity: " << str19.capacity() << endl; cout << "-----------------------------------------------" << endl; string str20; str20 += sz1; string str21; str21 += sz2; cout << "str20:" << str20 << " length: " << str20.length() << " size: " << str20.size() << " capacity: " << str20.capacity() << endl; cout << "str21:" << str21 << " length: " << str21.length() << " size: " << str21.size() << " capacity: " << str21.capacity() << endl; cout << "-----------------------------------------------" << endl; string str22; str22.assign(sz1); string str23; str23.assign(sz1, 9); string str24; str24.assign(sz1, 15); cout << "str22:" << str22 << " length: " << str22.length() << " size: " << str22.size() << " capacity: " << str22.capacity() << endl; cout << "str23:" << str23 << " length: " << str23.length() << " size: " << str23.size() << " capacity: " << str23.capacity() << endl; cout << "str24:" << str24 << " length: " << str24.length() << " size: " << str24.size() << " capacity: " << str24.capacity() << endl; cout << "-----------------------------------------------" << endl; string str25; str25.assign(sz2); string str26; str26.assign(sz2, 4); string str27; str27.assign(sz1, 15); cout << "str25:" << str25 << " length: " << str25.length() << " size: " << str25.size() << " capacity: " << str25.capacity() << endl; cout << "str26:" << str26 << " length: " << str26.length() << " size: " << str26.size() << " capacity: " << str26.capacity() << endl; cout << "str27:" << str27 << " length: " << str27.length() << " size: " << str27.size() << " capacity: " << str27.capacity() << endl; cout << "-----------------------------------------------" << endl; printf("%s\n", str26.c_str()); printf("%s\n", str26.data()); return 0; }
現總結如下：
1. 將char*賦值給std::string如果不指定長度，則會預設以\0截斷（ASCII碼值為0）；如果指定的長度超過char*字串的長度，用std::cout系列的函式輸出時，會把不屬於該char*之後的記憶體值打印出來。而對於printf系列的函式來列印std::string時遇到\0也會截斷，因而不能完全顯示。這點在列印或輸出到日誌時，這類字串時需要特別注意。
2. 如果是單個字元和一個字串賦值給std::string寫法是有區別的 —— 對於char，數目是第一個引數，對於char* 數目是第二個引數。
例如，假定ptsr是一個字串或者字元。如果pstr是一個字串，那麼要寫成string(pstr, n)，如果pstr是一個字元，那麼要寫成string(n, pstr)，而此時string(pstr, n)是一個錯誤的寫法，可能會導致你的程式產生莫名其妙的問題，因為如果pstr是一個負值，負數轉換成無符號整數size_t型別，n將非常大，會導致構造字串時length非常大導致std::string構造時丟擲異常。看這兩種初始化的建構函式簽名：
string (const char* s, size_t n);
string(size_t n, char c);
請注意這個區別！！特別容易想當然地寫錯。如果pstr對應數值比較小的正數，可能不會產生異常，但其實程式狀態也不是你想要的。如果是負數，程式將直接產生異常。丟擲：std::length_error 。所以出現這個異常資訊，你就要好好檢查下你初始化std::string的長度資訊了。