How to solve C++ syntax error: 'expected unqualified-id before '<' token'?

How to solve C syntax error: 'expected unqualified-id before '<' token'?

In the development of C, we often encounter various errors. One of the common errors is 'expected unqualified-id before '<' token'. This error usually means that an identifier is missing somewhere, but the compiler found the '<' symbol. This error is common when using templates or generic programming.

In this article, we will discuss how to identify and resolve this syntax error and provide some examples of common mistakes.

1. Reasons for the error

Usually, the 'expected unqualified-id before '<' token' error is caused by two common problems:

1. Missing template parameters
   When using template programming, you need to pass in appropriate parameters to the template. This error will occur if no parameters are passed to the template, or if incorrect parameters are passed.
2. Missing semicolon
   In some cases, this error will occur if there is no semicolon at the end of the statement.

2. Error examples and solutions

1. Example 1: Missing template parameters

template<class T>
class MyClass {
public:
    T value;
};
  
int main() {
    MyClass<int> obj;  // 正确用法：给模板传入适当的参数
    MyClass obj2;  // 错误用法：未给模板传入参数
    obj.value = 10;
    return 0;
}

In the above example, in the definition When using the MyClass class, you need to pass it a template parameter <class T>. When creating object obj, we successfully passed an appropriate parameter <int> to the template. But when creating object obj2, we did not pass in parameters to the template. In this case, the compiler will report an error and prompt 'expected unqualified-id before '<' token'. To solve this error, we only need to pass in a suitable template parameter to the obj2 object, such as <int>.

Solution to Example 1:

MyClass<int> obj2;  // 给模板传入适当的参数

2. Example 2: Missing semicolon

#include <iostream>
  
int main() {
    std::cout << "Hello World" << std::endl
    return 0;
}

In the above example, in the output statement std: :cout << "Hello World" << There is no semicolon after std::endl. In this way, the compiler will prompt the 'expected unqualified-id before '<' token' error. To resolve this error, we simply add a semicolon to the end of the statement.

Example 2 solution:

std::cout << "Hello World" << std::endl;  // 添加分号

3. Summary

In the development of C, the 'expected unqualified-id before '<' token' error is a common Grammatical errors. It's usually caused by missing template parameters or missing semicolons. By carefully checking the code and adding appropriate parameters or semicolons where the problem is, we can successfully resolve this error.

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